Tube amps have their voltage stepped down at the output of the amp through an output transformer. The best of the best tube amps have very expensive custom output transformers to minimize any losses. The output transformer on many designs is the bottleneck in the system. If done properly, these designs can be quite wonderful.
An OTL or "Output TransformerLess" amp does not use such a device and steps down the voltage through a series of output tubes. The advantage is this avoids any possible coloration caused by a transformer. Possible disadvantages are speaker matching (they like constant loads) and heat (they run in Class A).
OTL's are unmatched with the exception of very few transformer coupled amps (which cost $$$$$).
