Full Idea
There are two major kinds of externalist theory in the literature - probabilism (which expresses justification in terms of probability of the belief being true), and reliabilism (which refers to the probability of the cognitive processes being right).
Gist of Idea
Externalism comes as 'probabilism' (probability of truth) and 'reliabilism' (probability of good cognitive process)
Source
J Pollock / J Cruz (Contemporary theories of Knowledge (2nd) [1999], §4.1)
Book Reference
Pollock,J.L./Cruz,J: 'Contemporary Theories of Knowledge (2nd)' [Rowman and Littlefield 1999], p.91
A Reaction
A useful clarification. Reliabilism has an obvious problem, that a process can be reliable, but only luckily correct on this occasion (a clock which has, unusually, stopped). A ghost is more probably there if I believe in ghosts.