If you deactivate your account from the "Deactivate Account" section on the Account page, your profile and all information associated with it are immediately made inaccessible to other Facebook users. What this means is that you effectively disappear from the Facebook service. However, if you want to reactivate at some point, your profile information (friends, photos, interests, etc.) is saved, and your account will look just the way it did when you deactivated if you decide to reactivate it. Many users deactivate their accounts for temporary reasons and expect their information to be there when they return to the service.
If you do not think you will use Facebook again and would like your account deleted, this can be taken care of for you, but keep in mind that you will not be able to reactivate your account or retrieve any of the content or information you have added. If you would like your account permanently deleted with no option for recovery, please submit your request here. (see link below and select 'I want to permanently delete my account'.