Who uses it: Statisticians
What it means: The percentage of a sample that is likely to include all features of the population being tested. The confidence interval is what gives pollsters their margin of error; I think what that means is that a margin of error of five percent is the product of a confidence interval of 95%. (If I'm wrong, someone please correct me... statistics baffle me.)
How you can use it: As a measurement of how sure you are.
Yesterday was the worst of all possible weather: a cold rain, which melted the snow but froze into an invisible skin of ice overnight. This morning is foggy but warm, and I saw a female cardinal at a feeder up the street. Dizzy chased two fat squirrels, who were alert enough to escape him easily.
Maybe I shouldn't let Dizzy chase squirrels, but I figure the odds of him catching one are astronomically low. He's lame, and not the smartest -- this morning the squirrels had dashed over into the next yard, while Dizzy was still standing at the base of the tree where he saw them. But it exhilarates him, and it's good for the squirrels not to get too complacent.