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).
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.