From what I have read, formatting will delete any pics on the card.
Also found this if it helps.
Formatting a card basically insures that it is going to work right with your camera.
Just like computers, memory cards have files systems, folders, etc. When you use your camera to format a card, it sets the card up the way the camera needs it to be in order to function properly.
Now, for the most part, if you just pop in the memory card it will usually work. Most cameras will automatically put all the necessary "stuff" on the card for you, regardless of whether you've formatted it or not.
However, formatting a card is a very quick and (mostly) painless procedure, so I recommend doing it the first time you pop the card into your camera. Just in case. Since the actual way you format the card varies widely from camera to camera, you'll have to consult your owner's manual for specific how-to advice.
In addition to getting the card ready to use with the camera, formatting can also fix minor problems. If your card and camera are not getting along too well—lots of error messages, bad writes, etc.—try formatting the card. Sometimes it works, sometimes the card has passed to the great digital beyond, but it's worth a try.
All that useful tech stuff aside, formatting is a quick way to delete all the images off of a memory card. Rather then scrolling through and picking them off one by one, just do a format - the images are erased in a matter of seconds. Of course, don't do this if there are images on that card you want to keep :-)