Interestingly enough, this section of iCloud's settings can even refer to data that you saved only on your Mac. In my case, it revealed several megabytes' worth of content I'd saved in Preview while exploring; deleting those old, unneeded documents freed up a little extra iCloud space.
In addition, this section lists synced data from iCloud-leveraging apps and games. On my iPhone, that list includes Pages, Passbook, Keynote, and The Incident. I want my progress in The Incident synced between devices, and the sync data consumes just a single kilobyte, so it survives the cut. But if, for example, you see that Pages is taking up many megs (or more), you can tap it to get a listing of each document saved there, along with the amount of space it's taking up. If you notice files you no longer need, clean 'em out.
If you use iCloud as your primary email account, you may have your work cut out for you: All that mail counts against your iCloud quota. If you don't regularly archive email offline, prune attachments, and delete what's deletable, Mail may gobble up an increasingly large percentage of your iCloud quota. I don't use iCloud Mail, but I found that it was--at first--claiming about 20MB of storage space. I logged in to iCloud.com and cleared out my initial welcome message and a few other test messages to whittle that number down, though it never hit zero: I do use my iCloud account for syncing Notes between devices, and that data counts against the Mail tally, too.
Freedom can be free
Combine these tricks--disable Camera Roll backups, curate which apps get to back up their data to iCloud, limit what types of data you sync with iCloud, and prune unneeded iCloud mail--and you can dramatically shrink the space required to back up your devices with iCloud.
But if you just can't trim enough iCloud-stored data to fit everything you need, Apple is always happy to let you pay for more space.