In elementary school, a science fair is where kids get to show off all the great "science projects" that their parents did.

In high school, a science fair is where students actually do research into an area that hasn't really been researched that much before. These get won by people such as 14 year old high school seniors that call themselves Neutrino Girl, like in the 1999 Intel Science Talent Search.

Many times participation in science fairs is mandatory.