to format a disk means for an operating system to write a file system to the disk that the operating can use to store data.
What a format is doing is preparing a media card (for example) to be compatible with the operating system as many operating systems need this compatability to exist before it can be used.
The file system is a means for recording where data can be placed on the storage medium and to record where data is, so the operating system can call on the specific files when required.
When an operating system formats a media card, all the data on it will be lost as it will be written with a new file system.
Im not sure what file system is used on teh bold, but if you formatted the card in windows, the data will still be there, but the file system will not know about it. It will report the space that data is in as "empty" to be used for storing data.
when you copy data to teh card, the card will put the data in one of these "empty" spaces, ergo - overwriting the data.