I can't exactly explain so here are some example sentences:
What does it look like? (Asking for it's appearance in detail)
How does it look? (Asking for a more general response, like good or bad. Typically used when you want to know the state of something.)
For example, if I put on a dress and asked my friend how it looks, she might say "The dress is wrinkly but it fits nice." But if my friend told me she bought a dress and I asked what it look liked, she might say "It's white with red polka dots, and it goes down to my knees."