verbs - "To hear" or "to hearing"?

I often see constructions like this one:

I look forward to hearing from you soon.

It seems a little strange to me. In my mind it would look better using the infinitive form "to hear". I don't know if it has something with the verbals... Anyway, what I want to know is which form is correct and why?

Answer

This is not an infinitive. "Hearing from you soon" is a gerund, which functions as a noun.

I look forward to [hearing from you soon].

is the same kind of construction as

I look forward to [my vacation].