Virtue epistemology addresses the question of how we rationally justify knowledge. The way we justify knowledge is expressed in terms of intellectual or cognitive virtues, these virtues are normatively valuable, they warrant our approval.
By moving discussion about justification to a normative debate, virtue epistemologists reject a purely cognitive or positive approach to the problem of knowledge. Intellectual agents and groups of agents become the important source of epistemic value and therefore of knowledge justification, as opposed to reliance on, for example, sense perception or a priori truths.
There are a number of different schools of thought within virtue epistemology, the different approaches placing a greater or lesser emphasis on different cognitive or intellectual traits and epistemic norms.
Reliabilism concentrates on cognitive faculties which result in knowledge. Beliefs are warranted if they are formed by a process that generally produces true beliefs rather than false ones. A reliabilist approach tends to focus on the outcome of the process, if it is generally successful (reliable) then it is a good way to acquire to knowledge.
Responsibilism concentrates on intellectual character traits such as attentiveness, fair-mindedness, open-mindedness, intellectual tenacity, and courage. Responsibilists tend to concentrate on the approach taken by the agent to acquire knowledge. If the agent demonstrates laudable epistemic intellectual character traits then the result can be justified as knowledge.
For Sosa, intellectual virtue / virtue epistemology can be used as a way to reconcile or overcome the issues raised by Agrippa's trilemma. For example, virtue epistemology can overcome the objection to a purely coherentist approach where there could be a disconnect between a set of coherent beliefs and the external world.