‘Principle’ and ‘principle’ are often confused but do not, in fact, share any meanings.
‘Principle’ is only a noun and usually refers to a rule or standard. ‘The principles of socialism’, for example, refers to the tenets of the socialist ideology. ‘My principles prevent me from taking such petty action’ suggests that the speaker’s moral/ethical stand is against the action being spoken of.
‘Principal’, on the other hand, is both a noun and an adjective. As a noun, it has special financial and legal connotations, but in general usage it refers to someone who holds a high position or is important in a certain context: ‘a meeting of all the principals involved in the deal’. As an adjective it has the sense of ‘most important’: ‘My principal concern is to get my health back’.