Like other remote backup services it lets you backup your disk to their remote server. What really distinguishes this however is that you can also backup to any friend who is also running Crashplan. Come full-disk restore-time that can be a massive time saver since you can just drive over to your friends house, pick up the disk, and restore your hundreds of gigabytes locally.
Crashplan seems to have only one defect in my opinion. There's no way to turn it off temporarily! You can uninstall it, but that loses all your settings and possibly deletes the backups. You can set its back up times to be none, or you can set it to pause, but that does not stop the application from running, and since it's Java it can be a bit bloated in memory usage. (The reason they don't want you turning it off is because this not only stops your backups to your friend but also your freind's ability to backup and restore from your disk.)
So how do you turn it off temporarily?
Use Launchctl like this:
sudo launchctl stop com.crashplan.engine sudo launchctl unload /Library/LaunchDaemons/com.crashplan.engine.plist
sudo launchctl load /Library/LaunchDaemons/com.crashplan.engine.plist sudo launchctl start com.crashplan.engine
[crarko adds: I haven't tested this one. I use a few of the online backup services but Crashplan hasn't been one of them. Feel free to share your experiences in the comments.]

