Specifically, it's a subordinating conjunction, which means it introduces a dependent clause that modifies the main clause. It expresses a condition or reason.
Here's an example:
* As long as you study hard, you'll do well on the test.
In this sentence, "As long as you study hard" is a dependent clause, and "you'll do well on the test" is the main clause. The entire sentence makes sense because the dependent clause provides a condition for the main clause to be true.