Belief is a noun - the name of a thing, or (as in this case) an idea. Its plural is beliefs.
To believe, on the other hand, is a verb - it is something that you do. The 3rd person singular is believes ("She believes my assignment is all right.")
The two -s forms are pronounced differently - as they look. The noun has an unvoiced final consonant, IPA: /f/; the verb has the [[voice (phonetic)|voiced variant, IPA: /v/.
- For an etymological note about the origins of this word, see be-.