Both летать and лететь are imperfective.

That is a part of the big topic of Russian verbs of motion.

In addition to aspect (perfective/imperfective) verbs of motion can be unidirectional (лететь, идти...) and multidirectional (летать, ходить...).

I recommend you to google for "Russian verbs of motion" and then select whichever explanation you like most.