Hat-tip to Old Roommate for telling me about this website of spurious correlations.
For the non-statistical: a correlation is a mathematical relationship between two measures, that ranges from -1.0 to 0.0 to 1.0. The further from zero the relationship is (the correlation), the larger the mathematical relationship. A correlation of 0.0 means that there is no discernible relationship: any connection is totally random. A correlation of +1.0 means that the two things are completely locked together. And a correlation of -1.0 means that as one thing increases, the other perfectly decreases -- and vice-versa.
The human tendency is to think that these two things might be causally related. But it ain't necessarily so.
Two examples from the website: