The format function is used to create a new filesystem on a partition (and consequently, delete any existing filesystem).  To format a partition you must first select it in either the partition diagram or the partition list (when the selected item is empty space, this function is disabled).  Before formatting a partition, you must commit any outstanding changes using the write changes function, or else undo your changes using the undo function.  Only some filesystem types (FAT, EXT2, and Linux swap for example) can be created using the format function.  A dialog box will prompt you for the new desired filesystem type.  Once a format commences the data is physically changed on disk, so you cannot subsequently use the undo function to undo a format operation.