A very good example is SpiderOak, which in addition to offering the possibility of backing up your files to an online hard drive also lets you sync directories between several computers and share files in the cloud. To start using SpiderOak, you must have a user account, which you can create when you install the program and then verify via email.
Once you've set up your account, you can access the program's different services via the interface: backup copy, cloud preview, transfer status, shared file preview, and directory synchronization.
The backup system is simple. Just select which directories you want to safeguard. The program will then take care of uploading the files to the online storage drive and managing all the changes in files and directories that were backed up previously.
The file synching system works differently. Its job is to sync directories between several computers with SpiderOak installed. Thus you'll only be able to select and sync the directories you've already configured with each installation of the program. You can also sync directories belonging to the same computer.