Belief is the mental state in which an individual holds a proposition to be true. Beliefs are often metaphorically referred to as maps, and are considered valid to the extent that they correctly correspond to the truth. A person's knowledge is a subset of their beliefs, namely the beliefs that are also true and justified. Beliefs can be second-order, concerning propositions about other beliefs.
"I think it's an attitude, which is a set of dispositions to think and believe (and thus act) in certain ways. The disposition can be represented internally as a belief, but it's actually something more fundamental. The belief corresponding to an attitude is a representation rather than the thing itself." -- http://lesswrong.com/lw/7k/incremental_progress_and_the_valley/5cd#comments
"I think it's an attitude, which is a set of dispositions to think and believe (and thus act) in certain ways. The disposition can be represented internally as a belief, but it's actually something more fundamental. The belief corresponding to an attitude is a representation rather than the thing itself." -- http://lesswrong.com/lw/7k/incremental_progress_and_the_valley/5cd#comments