Full Idea
Internalism about justification says that the reasons one has for a belief must be in some sense available to the knowing subject, ..while externalism holds that it is possible for a person to have a justified belief without having access to the reason.
Gist of Idea
Internalists say the reasons for belief must be available to the subject, and externalists deny this
Source
Paul O'Grady (Relativism [2002], Ch.4)
Book Reference
O'Grady,Paul: 'Relativism' [Acumen 2002], p.102
A Reaction
It strikes me that internalists are talking about the believer being justified, and externalists talk about the belief being justified. I'm with the internalists. If this means cats don't know much, so much the worse for cats.