What does the capital E in red on some songs?

Explicit means it contains the song as actually recorded. Some songs are also marked Clean (a "C" in a box) which means that potentially offensive words have been removed by means of vocal suppression.

Hardly anybody actually buys the Clean version, but they are there to make parents of young kids feel good about the iTunes Store.

That is the Explicit symbol

Also this is something you can remove from iTunes in case you don't like it.

check this link:

http://www.wikihow.com/Add-Explicit-or-Clean-Tags-in-iTunes-with-Mp3tag-(Windows )