If I backup my Photos to iCloud, can they be restored in the albums they are sorted into on my iMac?

Time Machine backups seem to be available without using the Time Machine app. Here is an example of using Finder to examine one of the data files on a TM backup from August:

This is from opening the Picture folder in my users account. I can open the Photos Library packages. It's like being there, but without the cool Time Machine interface.

The iCloud Photos Library is not a back up, it's a sharing service, and it is possible to delete items both from your Macs library and the Cloud version inadvertently. The iCloud Photo Library is designed as a sharing service - as way to get your images on to all your devices.

Most Simple Back Up:

Drag the Photos Library from your Pictures Folder to another Disk. This will make a copy on that disk.

Slightly more complex: 

Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.

Example of such apps: Chronosync, SuperDuper, Carbon Copy Cloner - but there are many others.

Another backup option is the app Time Machine that's already in your Mac OS. It's an incremental backup like Yer_Man describes, but it's totally automatic and transparent. (The others can be set up like that, too.)

But the problem with most backups is that, if you back up a mistake, then your backup is worthless. Time Machine lets you restore Photos, for instance, to how it was an hour ago, yesterday, or the day before, or last week, or last month, as long as you've been using it. This is very powerful, but you need a reliable external drive about twice the size of your computer's hard drive. It doesn't have to be super fast like a solid state drive, but it shouldn't be a flash drive.

Richard you're right that Time Machine is available on the Mac already but I've left it out of the list as I'm not sure it's transparent. After the first back up it packages the increments in its own format, and you cannot restore the material without actually using Time Machine, as only TM can parse the original and the increments to get back to where you were. Hence I don't include TM.

The apps I list store the the library without changing the format of the back up. It's still perfectly readable from the Finder. So you don't need the app that made the back up to restore it. If you do want to use the app then they also include concepts like Snapshots, which mimic TM's ability to recover to a specific date. On the downside, they are software you pay for, but they do present more options.

Can you drag one of those libraries to another disk and open it?

Absolutely. Double click and it opens in Photos.