A countertenor voicebank is a male voicebank that can sing the higher octaves without distortion. The typical voice range of a counter tenor is A3 to C5 (or sometimes E5). They are capable of singing in the same ranges as alto singers, and occassionally the ranges of mezzo-sopranos. It is rare, but also possible for a countertenor to sing the part of a soprano.
The countertenor voice is often described as a light, agile voice.