Hi lucash. Welcome to English Forums.

Sometimes they're used interchangeably.
"What do you think of X?" (could be a person or a thing) Usually asking for a negative or positive opinion - for or against.

"What do you think about X?" Usually asking for more detailed information.
"I'll have to think about it." (I need to spend some time considering the matter.)
"I can't think of a single reason to hire that guy!"
"I just can't think of the name of that song!" (I can't remember/bring to mind)
- A.