In the decade after the death of Socrates, Antisthenes was the most important Athenian philosopher. Like his master, he tried to find out what words mean, but he was convinced that it was not possible to establish really good definitions (which was to bring him into conflict with Plato).
He did only partially agree with Socrates that someone who knew what was good, would not do a bad thing. Antisthenes added that one also had to be strong enough ("as strong as Socrates") to pursue what was good. Therefore, Antisthenes recommended physical training of all kinds, and wanted his students to refrain from luxury.
His most famous pupil was Diogenes of Sinope. Both men are mentioned as founders of the Cynical school.
A biography was included in the Lives of Eminent Philosophers by Diogenes Laertius (here).