Ooh, a newbie! Welcome to my kingdom, mate. Okay, the glitches...
*ahem*
Glitches are essentially bugs in the programming of a game. They occur completely by accident, and the results can vary, from amusing to dangerous. Many of the glitches in SSBM waver within the amusing sector, with additional characters, items, arenas and such like being available. One of the easiest SSBM glitches to use is known as 'Junk food'. It involves eating at such ab rapid rate that everything within a close vicinity is eaten. Basicaly, fling a Party Ball next to an item. If food comes out, start eating it while gradually making your way towards the item. If done succesfully, the item should be eaten. The results can vary, from an automatic long-range shot (Ray gun) to spontaneous combustion (Bob-omb). This is a fairly harmless glitch, and has only once yielded dangerous results. Mind you, that was when I got Samus to eat the Crazy Hand, so that's probably why.
10000 posts at last! w00t! Okay, I'm off into a quiet retirement. Don't bother me.