Google Play Services come pre-installed on just about every Android phone (well, except those sold in China). They are an essential mix of high and low-level stuff, everything from contact syncing to geo-positioning goes through these Services.
Play Services became the first mobile app (if you can call them that) to reach 5 billion installs. Thats 5 with nine zeroes. Granted, a good chunk of those are probably on devices no longer used, but as we said, there are also Androids not included in this count.
Whichever way you look at it, it is an impressive number. And in a sense, Google Play Services are what defines Android as we know it in the West many apps call on Play Services for vital functions rather than rely on the underlying OS.
This allows Google to bring new features to older devices because it can push updates to Play Services in the same way as with ordinary apps. As you well know, OS updates are not as simple. On the downside, if youre outside of Googles garden, then suddenly a lot of apps stop working something of a leash on smartphone makers.
Source | Via