Formatting a memory card
The process of initializing the memory card with a blank set of a system data is called "formatting". This initializes a memory card so that it can be used to store files. From the user's point of view, formatting a blank media does nothing; formatting a full media erases all the files on it.
On a PC, if you format a disk, a memory card, or whatever else, the data is typically recoverable. This is not always the case with a digital camera.
On a PC, formatting only writes blank set of system data onto the media, leaving user data (most part thereof) intact. Once the media is formatted, this data can still be retrieved using a data recovery program, process known as "unformat". To delete data irreversibly on the PC, you need to ask for it specifically, and use a special program, which overwrites each bit of the data with zeros. This process is known as "wiping", and some call it "low level format" (technically incorrect, but still a widespread term).
In Windows Vista and Windows 7 if you don't select "Quick format" then during the format data stored on the memory card will be deleted irreversibly and completely as in a "low level" format used in Canon Powershot cameras. After such Complete format data cannot be recovered at all. In Windows XP and earlier Complete Format didn't overwrite the data.
On the other hand, certain digital cameras would overwrite the entire memory card when formatting. This destroys all the photos on a card irreversibly. Canon Powershot series are most widespread example of a digital camera doing a "low level" format per its default settings. However, if you just delete one or several photos, these are (most likely) still recoverable using photo recovery software.
Continue to Accessing a memory card.