But is a conjunction. We use but to link items which are the same grammatical type (coordinating conjunction). But is used to connect ideas that contrast. I am allergic to strawberries. they still haven’t sold their house in London. The phrase but not is common:

The purpose of but is a conjunction, meaning "on the contrary; in contrast" (New Oxford American Dictionary). It is the same as and, but emphasizes contrast. You can replace it with "however" or "yet". In a comment, John Lawler wrote: It's just a conjunction. But means "and".