If you jailbreak your phone, the 'others' section will be big in size because thats where you install all the 3rd party apps from Cydia/Installer or movies/youtube/cycorder movies you download and not sync with itunes.

The "others" for unjailbreak phone also include some system files that resides in the /private directory i.e safari cache, maps cache, appstore temp files etc.

iPhone are partition into 2 partition which is Root partition for the system files and Mobile/Media partition for appstore apps/pics/musics/movies sync from itunes. The default for root partition on an unjailbreak phone would be 500mb. If you jailbreak the phone, you can increase the root partition to any size you desire. That why if you see the capacity in Settings --> General --> About is less than your actual capacity.