Deactivation hides your account, but your data is preserved in case you wish to return.
Deletion destroys your Facebook data. As a result you cannot recover it.
You can always backup your data then delete your account.
Here is Facebook's summary of the differences.
A deactivated account allows Facebook to keep your data. You are not adding to it, but it is still valuable to advertisers.
As a result, Facebook encourages account deactivation as opposed to full deletion of your data.