The plastic tripod you sometimes get along with the camera you purchased may not be a good tripod if it is not stable. Above everything else, a good tripod is steady. They are also extendable to greater heights. Good tripod can also be collapsed to stable low heights. They can hold heavier cameras as well.
Tripods are made in a variety of material, from plastic and wood to aluminum and carbon fibre (brittle, pricey). For normal use, aluminum tripods will do.
The best tripod: Vanguard Alta Pro (versatile, for low light photography or macrophotography, keeps the zoom lens steady)
The best affordable tripod: Slik Sprint Pro II (simple and light)
The best portable tripod: Joby GorillaPod (with twistable legs for woking in any environment, not for heavy cameras)