It can be used as:
* A subordinating conjunction: This is the most common use, introducing a dependent clause that modifies the main clause. For example: "I felt as if I had been asleep for a century."
* A coordinating conjunction: In this case, it connects two equal clauses, usually indicating a comparison. For example: "She's as tall as her brother."
* A preposition: It can also be used as a preposition, meaning "in the capacity of" or "in the same way as". For example: "He worked as a waiter."
It's important to consider the context to determine the specific function of "as" in a sentence.