Google has built in some pretty nice features into the Gmail app, as well as the service as a whole. Including their spam filters, Google Now cards, inbox prioritization and much more. To use these features, you used to need a Gmail account (or in the least a Google Apps for Education or Google Apps for Business account). However Google has changed that beginning today with the announcement of Gmailify.
Last year, Gmail added the ability to add other email accounts to the Gmail app. Those included Hotmail, Microsoft Exchange, and Outlook to name a few. However you didn't get the Gmail spam filters and other Gmail features in the app. Google notes in their blog post announcing Gmailify, that many users don't want to go through the hassle of switching email accounts - which can be a bit of a daunting task - and now you don't have to. As the Gmail team states, you can "Gmailify your inbox."
What Gmailify does here is that it links your existing account on into Gmail. This way you get all of the Gmail features while sticking with Hotmail, Yahoo Mail or whatever provider you currently have. To enable Gmailify, all you need to do is open the Gmail app and then sign in with your email account and then enable Gmailify. It's really quite simple. The Gmail team has also made it easy for you to unlink your account if for any reason you don't want to use these Gmail features any more.
This is all part of version 5.11 of the Gmail app on Android. This update is rolling out to the Google Play Store as we speak. Currently there is no APK available for you to sideload, but that should be appearing shortly. Like most Google updates, this is likely a staged roll out, so you should have it in a few days if you don't already. This update is definitely a great update for those that have to keep their other email accounts open, or just don't want to switch, but still get Gmail's fantastic features. The spam filters alone is definitely worth linking your account to Gmail.