Quote Originally Posted by JJ
It's easy, this is a verb of motion. Some examples:
Лететь - mostly to fly to, to fly from, летать - just "to fly".
Птицы летают в небе - The birds are flying in the sky/The birds fly in the sky.
В небе летят птицы - In the sky the birds are flying to (somewhere).
Птицы летят на юг - The birds are flying to the south.
Я умею летать - I can fly.
Я летаю - I fly (present simple) - Я летаю в Москву каждую неделю - I fly to Moscow every week.
Я лечу - I'm flying to/from.
yep

I have to add the following:
летать is used if someone has no target, like flying around etc.
whereas лететь understands that smbdy has a target, like he's flying to the north...