Seven string guitars are guitars that have an additional B string on the lower end, with the whole guitar being tuned B-E-A-D-G-B-E. 7 strign guitars are common in metal for getting a thunderous chug. Many great players have use them to great effect, like John Petrucci (Dream Theater).
You can play any 6 string chord on a seven string guitar when it's tuned to standard tuning. But with an extra string and no extra fingers, there are unique chords to be learned on a 7 string as well.