Specifically, it's a coordinating conjunction. Here's why:
* Conjunctions connect words, phrases, or clauses.
* Coordinating conjunctions connect items of equal grammatical rank (like two words, two phrases, or two clauses). They typically join elements that are similar in structure and function.
"But" is used to show contrast or opposition between the elements it connects.
Example:
* "I wanted to go to the beach, but it was raining."
Here, "but" connects two independent clauses ("I wanted to go to the beach" and "it was raining"), showing that the second clause contradicts the first.