Translation:I open the window.
Grammatically, あく can only be intransitive (the transitive form is あける); ひらく can be either intransitive or transitive.
In terms of the differences in usage, the following links explain the distinctions (the latter one has quite a few usage examples):
An excerpt of the second link:
The translation is both to open the door and there is time when we use them equally. But 開く = ひらく ( = hiraku) is a bit more formal, literal, a bit dramatic and is used when you stress the movement or describe the process of opening something “folded / closed tightly.” or spread something widely.
It also covers some idiomatic uses.