Rescue as part of search and rescue, is finding an overdue kayaker, and assuring they are delivered from whatever stress, injury, or catastrophe kept them from returning on time from a trip. This is sometimes called an "emergency rescue."
In extreme cases, the Coast Guard, a passing motorboat, Search and Rescue, or an act of God can rescue you. These are not truly "Kayak Rescues", since you often are too hypothermic (or have a trashed boat) in the event you need this type of emergency rescue, and you don't get back in your boat that day.
Rescue as part of being upside down in a kayak is different. To make the difference clear, call it a "kayak rescue."
Rescues are all about dealing with events that lead to capsize. The best technique is to avoid getting into trouble in the first place. This requires GoodJudgement. If that fails and you flip upside-down, your best bet is an Eskimo Roll - a technique that requires no additional equipment and is quick and safe.
If you can't execute a roll for some reason, you need to wet exit from your kayak and after gathering your wits, get back in. This is a rescue.
Some folks distinguish between a Recovery and a Rescue. A Recovery is when you use an Eskimo Roll to recover from a capsize or a brace to prevent the capsize in the first place. A rescue, on the other hand, is any technique that puts you back in your boat following a wet exit.
Rescues come in two flavors: a Self Rescue and an Assisted rescue. As the names imply, you either get your self back in your boat using your own skills and the equipment at hand; or, you use outside help--other members of your party.
Here is a list of techniques for dealing with a capsize event.