its probably an inverted chord. For example, the notes of an E major chord are E, G# and B, but if you played them in a different order, then you would have G#, B and E. This is an inversion (a 1st inversion in fact - a 2nd inversion would be B, E and G#, although the 2nd two notes can be in either order - it's all to do with what note is the lowest note). I think the best way to learn them is to look at a diagram (or draw one) showing which note each fret on each string is. Then, if you were playing an inversion of an Emaj chord, then look for where E, G# and B are on the diagram. For example:
E --------4-----------12--19-|
B ----5---5-------12--12--21-|
G 4---4---4-------13--13--21-|
D 6---6-------9---14---------|
A 7-----------11-------------|
E ------------12-------------|
These are all inversions of E major chords. You can move them up and down the neck to play them in different keys. Use the same technique for minor and 7th chords. Instead of playing all the notes, you can just play two of them. For example, E and G#, or G# and B. You don't necessarily need to include the root of the note, because if the bass is playing it then its not essential. Same thing goes for 7th chords - you can leave out any of the notes and it will retain its tonality to a certain extent.
