In today’s common usage, these two phrases are interchangeable. Both “as if” and “as though” can be used to express what something or someone seems like:

Leslie looks as if/as though she’s angry.

They can also be used to express something unreal:

Henry acts as if/as though he’s an alien.

Some sources make the distinction that “as though” should be used for more likely situations—Leslie being angry—and “as if” should be used for less likely situations—Henry being an alien. However, this isn’t an actual rule.