1. The most common 7th chords are commonly refered to as Major 7, Dominant 7, and minor 7. These all have a perfect 5th, which can be left out if you can only reach three notes in a chord. The 3rd and 7th tones, along with the Root, are sufficient to convey the type of 7th chord being played/heard.

2. When playing ii-V-I progressions, the simple type of voice leading shown on the previous web page has the Root moving in 5ths down (or 4ths up) while one voice on the treble clef moves and one does not move.

Notice in the first measure of the example, the Dm chord is voiced D-F-C (R-3-7), then the G7 chord is voiced G-F-B (R-7-3), then the CM7 chord is voiced C-E-B (R-3-7). So when moving from Dm to G7 the F is a held note, which is the 3rd of the Dm but becomes the 7th of the G7; meanwhile the C, the 7th of the Dm chord, moves down to B and becomes the 3rd of the G7 chord. The 3rd and 7th also flip when changing from the G7 to the CM7.

If this is new to you, there's a whole world of music theory just waiting to delight you. Find out more from a teacher, or a book, or one of the many web pages giving this info out for free.