The more expensive laser levels are self-leveling and others require the operator to adjust them. The great thing though, is that they are extremely accurate, even for long distances.

Bubble levels have been the standard for a very long time, but laser levels are getting cheaper and easier to use all the time, like this one from Ryobi.

A laser level can be an invaluable tool for keeping things straight, but which color laser level you use may be more important than you realize.