Google Docs is a cloud-based word processor owned by Google. It's free for anyone with a Google account to use. Since it's cloud-based, you can access it anywhere you have internet (and you can configure it to be available without Internet, provided you already have the document downloaded and available offline).
What It's Good At
Google Docs is perfect for the writer on the go. If you have Internet access through a laptop, tablet, phone, or anything, you can open your Google Doc and work on your writing. While you can work on your document offline, configuring this is tricky, and you will eventually need a good Internet connection to get all your changes synced with the online version of the document.
Google Docs does keep a revision history, so you can go back and re-open a previous version of a document, or see what changed.
It also comes pretty pared down. This makes it load fast, and eliminates some of the overpowered parts of Scrivener and Word that can feel confusing to use if you're not as familiar with word processors. There are add-ons you can get for free that let you customize Google Docs as you like.
Probably my favorite part is how easy it is to share the document. If I want a beta-readers' eyes on it, I can just share the document with them.
Where It Falls Short
What's interesting with Google Docs is how the things that are great about it are also the things that are...not so great about it.
For example, the cloud-based part of it is great or awful depending on your situation. And while you can work on things offline, figuring that out can be difficult for someone who isn't as familiar with the product or the interface. And if there's an unplanned Internet outage (A storm takes out my power, but hey, since I can't vacuum now I'll open my laptop and write until my battery dies...or not) you may not be able to do your writing. And while there are ways around this, when considering how it fits into the flow, it's both great and awful. It's great because I can open my document and do a little writing just about anywhere. It's terrible because I have to have an Internet connection to do my writing unless I really plan ahead.
The pared-down part is really frustrating for me, personally. I use Word a lot with work, so I know all the features it has. And while many of them aren't necessary for drafting a novel, finding an add-on to add the functionality I want to Google Docs is annoying. Granted, once I get my add-ons sorted, that annoyance goes away (sort of). But the way you use the add-on is often different and can feel clunky depending on how well the developer integrated it into Google Docs. Sometimes they're buggy, and sometimes they don't work like you think they will based on the description. I mean, it's free, so I guess you get what you pay for?
And finally, the ease of sharing also means it's a lot easier for someone you don't want to see the document to see it. And while you can take steps to mitigate this, it's still riskier than having a hard copy on your hard drive.
Overall, I give Google Docs 3.5 stars out of 5. It's a decent solution, and it's definitely affordable. The integration with other apps makes it easier to meld Google Docs with some basic plotting tools, like Sheets or Excel. For someone who values availability and ease over functionality and security, it's the perfect solution.