The hard cartilage of the ear directly above the lobe. It sticks out towards the tragus, helping to form a protective shield over the actual ear opening. An antitragus piercing looks like a slightly more complicated lobe piercing, but is in fact substantially more difficult and painful, since it is true cartilage, and fairly thick at that, as opposed to the soft flesh of the lobe. A benefit of an antitragus piercing is that it doesn't draw immediate attention, but if someone wants to notice it, it is clearly nonstandard, and forces the noticer to consider a part of the ear that most people aren't really fully aware exists.

If this is roughly what a left ear looks like, the antitragus is near the linked part. To get a better idea of where it is, though, you may need to feel the upper edge of your lobe until you find the bit that's harder than the rest, and sticks out just a little bit.

/ _  \
\_ \  |
 \  | |
 / _/ |