HIS is by no means exhaustive (I know: where's Zwingli?) but should, I hope, give a good overview.
It's true that because Protestant churches are now so different even from each other it's hard to generalise but I think all believe thus:
Individual conscience's understanding of the Bible is more important than church tradition, which is of secondary if any importance depending on the group. Which when you get right down to it is what sola scriptura really means, from 'Jesus loves me, this I know/'cos my pastor tells me so' (for many of them, 'the Bible says' really means 'my pastor says') to, ultimately, a magisterium of one: you. The church is primarily an invisible fellowship of believers individually saved - the institutional church or visible manifestation of that is man-made and secondary. Confirmation, ordination, marriage, confession/absolution and anointing of the sick are not sacraments of the gospel. Most practise the second and third, and many a form of the first, but don't count them as sacraments proper. Baptism and Communion are. (Lutherans do have confession/absolution but don't count it as a sacrament like the other two.) A true Protestant's belief about Communion ranges from mere symbolism - it's only bread and wine/grape juice; the only real presence is the worthy believer receiving Jesus in his heart including at Communion - to something like Catholic belief but falling short of it (not a complete and/or permanent change in the elements). When all other things seem the same - creed, sacrament, liturgy - Catholics believe in an infallible church; Protestants don't.
Communion service at an Anglican parish church in England, 1581
I think you can divide Protestantism into about four categories (four and a half if you count Anglicanism as Protestant).
1. There are the classical Protestants, the original groups that broke away around the 'Reformation', all credally orthodox Christian about the Trinity, etc. Such as:
Lutherans, who, ironically considering Martin Luther's place in world history as the father of Protestantism, are much less Protestant than most people think. I think in Germany historically for example it's obviously a reformist Catholic movement pushed over the edge. They're liturgical and sacramental - they believe the same thing about baptism as Catholics and nearly the same about the Eucharist only falling short of belief in a complete and permanent change of the elements (their view is functional - as long as you intend to use the elements as the sacrament, that's what they are). Some go to their pastors for confession/absolution. Many use the crucifix. Some cross themselves. And some are episcopal - they have bishops who claim apostolic succession (not recognised by Rome) but no one form of church government is considered essential. Faith vs works is just a big misunderstanding now being resolved between the two sides, Catholic and Lutheran. Most striking difference: most don't have bishops and thus don't claim to have priests. The 'highest' among them say they're not Protestants!
Calvinists (Reformed on the Continent, Presbyterians in Scotland, Congregationalists in England and America). The system of brilliant but wrong French layman Jean (John) Calvin including double predestination (if you were meant to be damned there's nothing you can do about it!), the total depravity of fallen human nature (among other main points), no bishops and denial of the Real Presence. Polity is through congregational groups of elders to presbyteries to synods IIRC. Like Lutherans they do infant baptism.
1.5. Anglicans could go in a category by themselves - half Catholic schism, half classical Protestant - as a vocal minority say they are not Protestants! The only definite thing you can say about the 'Reformation' in England is it was an act of state. King Henry VIII wasn't a Protestant at all. He was a schismatic. After he died the English Church became a weird mixture of things meant to please the king (queen) and as many of the English people as possible (which it didn't), to bring them as much under the king's control as possible, by having them all go to the state church. You ended up with a credally orthodox but very Calvinist church theologically... but with liturgical worship, especially the daily office (Morning and Evening Prayer), and with bishops who claim apostolic succession! (Which isn't recognised by Rome.) Classical Anglicans fall short of Catholic belief in the complete change in the Communion elements but unlike Lutherans hold that the change is permanent. Today Anglicans come in four versions called churchmanships: Anglo-Catholic (click the link above), Central, who resemble Lutherans but with bishops and belief in a permanent sacramental presence in Communion, Low (Evangelical, capitalised in England when describing them), who are like old-school Calvinist Anglicans and even conservative Presbyterians and old-time Congregationalists, and Broad, the modern liberals, some of whom are still Christian in their beliefs, others not (ranging from neo-pagan to atheist). Most churchmen except Anglo-Catholics accept the attempted ordination of women.
- The Methodists are an 18th-century breakaway from Anglicanism, a revival movement that moved away from Calvinism back Catholicwards towards a more optimistic view of fallen human nature and free will in being saved. They were essentially driven out of the English Church, even though their founder John Wesley was an Anglican priest who never left, and so don't claim to have apostolic bishops (but use the title of bishop).
2. Then there were the radical Protestants such as:
Anabaptists (rebaptisers) who denied infant baptism and saw the sacrament as just a symbol, not something giving grace. The German Mennonites and their offshoots like the Amish and Hutterites are this.
- The English and American Baptists aren't directly related to them. An English Congregational minister called John Smythe adopted this idea and started the Baptist church. Congregational polity and individual interpretation of the Bible are big in this tradition so you get a range from very conservative (Jerry Falwell, who was an independent Baptist most of his life, the Southern Baptist Convention, etc.) to very liberal (the American Baptist Convention, just like a mainline Protestant denomination).
The Quakers in England and then America, Christian in belief but not using the creeds AFAIK.
- The Shakers were an offshoot of them. As all members are celibate they've almost completely died off except about 15 people, mostly recent converts (?), in America.
Then in modern times there are:
3. The Pentecostal movement and churches, growing out of Methodism. Claims direct experience of the Holy Spirit with charismatic gifts like prophecy and healing, and that glossolalia (ecstatic nonsense speech) is biblical speaking in tongues (disputed by other Christians). Most are credally firmly, conservatively Christian but a few have apostasised to 'oneness Pentecostalism' which denies the Trinity and thus is no longer Christian.
4. The liberal/Modernist trend in the old mainline churches, away from credal Christianity and towards the vague belief or unbelief of the Unitarians (Congregationalists who apostasised from Christianity during the 18th-century 'Enlightenment'). The creeds and sacraments of baptism and Communion may still be there but as empty trappings - some liberal Protestants are still Christians, others not.
There are a few loose ends that need tying up:
19th-century end-of-the-world hysteria ('The world will end in 1844!' claimed a man called Miller) produced the Seventh-Day Adventist Church, essentially evangelicals (do they baptise babies?) who go to church on Saturday not Sunday and keep kosher (more SDAs do in the US than Jews). They are Christians. Unlike their Arian offshoot, the Jehovah's Witnesses.
There is also the Restorationist movement which sought to transcend denominationalism and restore the original primitive Christian church. A Presbyterian minister called Campbell started that IIRC. Predictably it branched off into denominations itself, including the liberal Disciples of Christ, the conservative Churches of Christ (big in Texas - they seem Baptist about baptism and ecclesiastical polity but have their version of Communion every Sunday) ... and the now apostate (henotheists - believers in plural gods but worshipping only one - they are no longer Christians) Mormons.
Update: Somebody has written to me:
As a member of the Churches of Christ I would say 1) the Restoration movement branched off into three major groups... and 2) your typical Baptist would choke at the suggestion that baptism as taught among the Churches of Christ is the same as taught in Baptist circles. Both groups do practice adult immersion but part ways in that among Churches of Christ it is believed that baptism is the point at which one is added to the church, something that is anathema to Baptists, but as I understand it similar to RC teaching.
- The Reorganized Church of Jesus Christ of Latter Day Saints, since 2001 the Community of Christ, was continued by descendants of Mormon founder Joseph Smith, who started his church as Christian but changed his mind. They accept his Book of Mormon alongside the Bible as scripture but hold to his early opinions and not Mormon doctrine. They are Christians, not