Is but a conjunction

Yes, the Word 'but' is one of the 7 coordinating conjuctions in the English language. They are used to join Words, phrases or clauses. The other 6 are: for, and, nor, or, yet, so they are the fanboys
Yes, the Word 'but' is a conjunction, a preposition, an adverb, and a noun.

EXAMPLES:

I never win, but I keep trying. (conjunction)

All but three students passed the test. (preposition)

He is but a simple fellow. (adverb)

I'm listening to your excuse. I'm waiting for the but. (noun)