If you do an exchange through one of the exchange companies such as RCI or II, it works sort of like a bank. You deposit your week into their bank and then you are able to draw out someone else's week. You don't have to wait until your week is taken. You can search online to see what's available.
You can also put in an ongoing request for a particular area, resort, or time period and the computer does the search for you. If what you want turns up, you'll be notified. It usually works to your advantage to do an ongoing search because some of the more popular exchanges never make it to the online search because they are snatched up by previously submitted ongoing searches.
If you own a points resort or system, it works like a bank, too. You have X number of points and you can use them to exchange for weeks or days in other points resorts based on the value that has been assigned for each resort. More desirable resorts/seasons/locations require more points. Last minute exchanges usually require fewer points.