Minecart

Minecarts are a type of movable container, crafted from iron. Minecarts will follow a path laid out by tracks (including sloped paths and curves) until they reach the paths end.

Currently, once placed, a minecart cannot be destroyed or picked back up. Should the cart become derailed, you may push it back onto rails and it will snap back into place.

Minecart Riding
Notch recently announced in his latest blog post that the next major update will include ride-able minecarts. He posted a video demonstrating minecart riding in action. Minecart physics are obviously being improved, since the video showed Notch's character riding on a rollercoaster-like course, and the minecart being able to, with sufficient momentum, ride up hills. It is yet to be determined if A) these minecarts will be different then storage minecarts, or if B) they will work off-rails.