Because they need Nitrogen to live, and they just can't find it in the soil, these wise and wily plants eat other buggers to get their fix.

Some of the most wild, and wacky looking species out there - some of them even eat FROGS.

The main different types are as such:

Pitfall traps of pitcher plants are leaves folded into deep, slippery pools filled with digestive enzymes.
Flypaper (or sticky or adhesive traps) of sundews and butterworts are leaves covered in stalked glands that exude sticky mucilage.
Snap traps (or steel traps) of the Venus flytrap and waterwheel plant are hinged leaves that snap shut when trigger hairs are touched.
Suction traps, unique to bladderworts, are highly modified leaves in the shape of a bladder with a hinged door lined with trigger hairs.
Lobster-pot traps of corkscrew plants are twisted tubular channels lined with hairs and glands.

Anyways, dig it, this is one of my favourite websites, far and beyond.
botany.org/carnivorous_plants/