When I was little, I knew of only two nail shapes: dagger (like that of my then mean older cousin — ouch) and bitten down to the nubs (like me — also, ouch). While you can wear your nails however you please, I've got some guidelines on choosing which nail shape might suite you best.

Generally speaking, try to match the shape of your cuticle to your free edge (the part you are filing). This will give you the most complementary finish. Another basic guideline is to shape the nail to give the illusion of elongated fingers. Remember to file nails in one direction, and check out some more tips below:

Oval — Looks good on most, particularly short, wide fingers.
Round — Great for short nails and those with very active hands, since round tends to break less.
Almond/Pointed — Elongates petite hands and fingers. It's also a very of-the-moment style, according to nail guru Deborah Lippmann.
Square — Works well to balance out long, narrow fingers.
Squoval — Softens wide nail beds. Also works well for short, wide fingers or long, big fingers.