A jigsaw is a hand held saw with a small base that allows you to cut straight lines or curves.
This example is a Freud FJ85 Heavy Duty Variable Speed Jigsaw. It features a 6amp motor, variable speed, quick blade changes, vacuum port and a non-marring base. Models like this one are capable of cutting differing materials for an extended period of time. Other jigsaws with less power are designed for smaller and more delicate work and even come with features that allow them to make more intricate cuts (using one of the jigsaws with less power for heavy duty cuts can burn out the motor).
When purchasing a jigsaw, select the model with the features that best fit your needs. If you are looking for something to do delicate, intricate cutting, consider a scroll saw.