B7 is a seventh chord and it is very often used because it is the dominant seventh chord of E-major which is one of the most often used keys on the guitar.

E9 is a ninth chord and a ninth chord is a chord with a ninth, and is an extended chord. Here the fingering is given so that you can use it to play all other ninth chords (X9). Just pick a right position.

Am9 is a minor ninth chord and belongs to the category of extended chords because it has more thirds than the seventh chord. What makes this chord a bit unusual here is that although it is played from the 5th position, it...

A major triad belongs to the simplest category of chords: it is a TRIAD so it consists of 3 different notes which are: LA-DO#-MI.