Basically, you tell the shelter you'd like to foster a dog. Then they'll give it to you to have temporarily until a forever home comes along. There's no adoption fee but sometimes you're required to pay for food (depends on the shelter I guess). I don't know if it's how it works there but over here, they foster out the ones that are really young, stressed or need lots of medical attention.

A few places will let you have a trial run too for a couple days. Fostering would give you a bit more time to see how the dog warms up though..since fostering tends to be longer.