A deed is the document that transfers ownership of real estate. It contains the names of the old and new owners and a legal description of the property, and is signed by the person transferring the property. You can't transfer real estate without having something in writing, which is almost always a deed.
Here's a brief rundown of the most common types of deeds:
A quitclaim deed transfers whatever ownership interest a person has in a property. It makes no guarantees about the extent of the person's interest. Quitclaim deeds are commonly used by divorcing couples; one spouse signs all his or her rights in the couple's real estate over to the other. This can be especially useful if it isn't clear how much of an interest, if any, one spouse has in property that's held in the other's name. (However, a quitclaim deed doesn't relieve the individual transferring ownership from the mortgage, if there is one.)
Quitclaim deeds are also frequently used when there is a "cloud" on title -- that is, when a search reveals that a previous owner or some other individual, like the heir of a previous owner, may have some claim to the property. The individual can sign a quitclaim deed to transfer any remaining interest.
A grant deed transfers ownership and implies certain promises -- that the title hasn't already been transferred to someone else or been encumbered, except as set out in the deed.
A warranty deed transfers ownership and explicitly promises the buyer that the transferor has good title to the property, meaning it is free of liens or claims of ownership. The transferor guarantees that he or she will compensate the buyer if that turns out to be wrong. The warranty deed may make other promises as well, to address particular problems with the transaction.
If you're transferring property in California, Deeds for California Real Estate, by Mary Randolph (Nolo), gives you step-by-step instructions on choosing the right deed, creating it, and filing it with your county recorder.