Reliabilism is an alternative theory of justification to foundationalism and coherentism. According to reliabilism, whether or not a belief is justified is not determined by whether or not it is appropriately related to other beliefs. Rather, according to reliabilism, a belief is justified based on how it is formed.
There are good and bad ways to go about forming beliefs. Beliefs based on reliable belief-forming mechanisms are likely to be true. Beliefs based on unreliable belief-forming mechanism are not. The reliabilist holds that a belief’s justification depends on whether it is formed using a reliable or an unreliable method: If perception is a reliable method for forming beliefs, then beliefs based on perception are justified. If wishful thinking is a reliable method for forming beliefs, then beliefs based on wishful thinking are justified. Conversely, if either of these methods of belief-formation is unreliable, then beliefs based on them will be unjustified.
Reliabilism is an externalist theory of justification. Whereas internalist theories hold that justification is determined by states internal to the believer, states to which the believer has infallible access, externalism holds that it is not. If we cannot know whether a belief-forming method is reliable, then we cannot know whether our beliefs formed in that way are justified; nevertheless, on reliabilism, if it is then they are justified, and if it is not, then they are not.