What is Keep?
It has a few tricks. It's generally considered a "note" taking app. Google Keep is collaborative. Google Keep is great for individual or shared 'To Do' lists. Keep can provide reminders based on time and date or location. Keep is mobile. Keep syncs with Google Calendar Reminders. There's also a handy Chrome extension that works great along with Gmail. And as of a few weeks ago, Google Keep is officially supported by Google as part of G Suite. So it's part of their business offerings and should continue to see investments from Google. If you're still using Google Tasks, I'd switch to Keep asap as it looks like this is probably the direction Google is heading. If you work in the Google ecosystem of apps, Keep is a solid addition to your toolkit.
What it's not
If you love Evernote or OneNote, Google Keep is probably not going to be a replacement for either one of those. I think of Google Keep as more lightweight than either of those services. Keeping quick notes, to do lists, reminders, etc. for yourself or a small Group and Keep can be a great tool. It also integrates nicely with other G Suite services like Docs and Gmail.
Checking Google Keep out from a personal or organization Google account is super easy. Just visit keep.google.com from your browser and you can also grab the Keep application from Google Play or the App Store.