Info-Mac

Why can't I share music or use them as ringtones? And when people send me music thru mms or something I can't use it or move it to my playlist or ringtones?

Dunno, but you definitely can if you sync to your computer and email them to yourself.

"Why can't I share music?"

You can, within your own home network. Check out "home sharing."

If by "share" you mean "redistribute for free to anyone I happen to know," you can't do that because it's illegal.

"Why can't I use (music) as ringtones?"

You can. There are a wide variety of ways to do this for "free" (not counting time and effort), "cheap" (third party options) or "cheapest" (Buy the ringtone directly from iTunes. Yes it's another whole dollar. Wooooo.)

"When people send me music through mms or 'something' ..."

Probably illegal. You may not wish to obey the laws regarding copyright and just compensation to the artists, but Apple doesn't have that option. Nonetheless, depending on the format of the music being sent, you could indeed move it into iTunes (and thus playlists) or edit yourself a ringtone with it (see above).