A snap is where you pay $69 in hopes of capture a domain name when it drops. Snapnames has agreements with over 15 registrars to use their available resources to attempt to register domains when the domains drop, giving them the advantage over any other service. Snaps can easily be "exchanged" to a different name if another service happens to win the domain.
Pool.com is where you pay no up front fees to backorder a name. If they are successful in capturing the domain name you pay $60, or if more than one person has backordered the same name there will be a 3 day auction available only to those people who had the domain on their list.
A backorder is a service by godaddy that also attempts to register a name when it drops. They are cheaper than snaps at $18.95, but are more commonly unsuccessful. You can easily change the domain you backordered, if it should not be successful.
