The KT88 is a beam tetrode/kink-less tetrode (hence "KT") AF Valve.
Fits a standard eight-pin octal socket and has similar pin-out and applications as the 6L6 and EL34.
The KT88 segment is a hotly contested market at the moment. Brands include Genelex/Gold Lion, Winged C, Sovtek and Valve Art.
KT88's are supplied as matched pairs and quads or sextet and octets by special order.