Google Keep is the latest of Google's countless free productivity offerings. It's an app that lets you write notes, or record voice memos, for your later perusal. Like any memo app, it's great for reminders or for long-term plans both. You can also collaborate in real time with co-workers, friends or family.
Work together with friends, but maybe not with other apps
The basic interface of Google Keep is very robust and lets you create, type and organize memos easily. Though some of the memo organization features are a little weak, for any small- or medium-sized collection of notes, they should be enough. Those with huge numbers of memos might consider alternatives or supplements to Keep, though. Keep also integrates with Google's other services, but not seamlessly: it only has partial compatibility with Gmail for example, and lacks even some of Gmail's organizational features as well.
Keep in touch
Overall, Google Keep is a fairly mature piece of software that does what you'd expect it to, and does it well, with the guarantee of quality that you get when you use any Google product. Though as of this writing it's missing a few desirable features, like pinning notes, new features are frequently added, and this is a fantastic first choice if you are looking to try note-taking apps.