Having trouble transfering my music between computers HELP!!?

So I've bought a new WD Elements 2Tb external hard drive so that i can transfer my music from iTunes on my old computer to iTunes on my laptop. My issue is that when copying and pasting my music out of iTunes and on the hard drive, some of the music has a Windows Media Player thumbnail and some has a Quicktime Movies thumbnail. Why are they different? Is that just the difference between MP3 and AAC?