Do m4a files work anywhere, or only on Apple devices?

I'm trying to copy music files from my home external HD where my iTunes library lives, over into a portable external HD I'm taking on a trip.

I will want to listen to music in a car with my EHD connected to the car's USB port.

Some of my music seems to be m4a files instead of mp3 files. I don't know what they are. Will they play away from iTunes?

Thanks,

Karen