Let's say -- I am, for the record, stealing this from Wikipedia -- you live on a block that is suffering from a lot of breaking and entering.  Your neighbor suggests everyone on the block pitch in to buy a CCTV system.  It would cost £2500, or £100/person.  Everyone else might agree to this, but you might not, since you know that the syystem will protect you regardless of your decision on whether or not to throw in that £100.

In that case, you have a free-rider problem -- you, of course, being the free rider in this example.  In the case of a charity, people who refuse to donate, but still enjoy the benefits (often positive externalities), are free riders.

Conservatives want live babies so they can raise them to be dead soldiers. - George Carlin