Look here for details on photo stream.
On the other hand, any videos in the camera roll are backed up to icloud (along with photos) - see link below. But the only way to sync video to other devices is to rely on itunes and USB. Considering the number of people who have lost photos doing a restore from an icloud backup, I'd say - don't rely on icloud for backing up photos and videos.
Apple doc on backup and restore on icloud:
BTW, icloud is not a storage service like Dropbox, it's primarily a syncing and backup (for iOS devices) service. So don't expect to offload files to icloud to save space on your phone. As for backups, yes, you can choose what you want to back up. Go to...
Settings>iCloud>Storage & Backup>manage Storage, tap my device name in the Backups section, then look under Backup options