MUST SEE: Different Android O.S Versions

The list of differences between the Jellybean, Kitkat and Lollipop is enormous.
I’ll mention some of the major differences between all of them in chronological order :
First of all let us look at the Android version numbers Gingerbread onwards as it was then that Android slowly started becoming popular.
When you place the screenshots of each homescreen you can notice the relation between the UI changes and the version numbers

Gingerbread (2.3 —> 2.3.7)

Honeycomb (3.0 —> 3.2.6)

Ice Cream Sandwich (4.0 —> 4.0.4)

Jellybean (4.1 —> 4.3.1)

Kitkat (4.4 —> 4.4.4)

Lollipop (5.0 —> 5.1.1)

When you place the screenshots of each homescreen you can notice the relation between the UI changes and the version numbers.

• A.x.x to B.x.x —> Indicates a major UI overhaul along with a bunch of new APIs and features of Android which developers and end users can work with. Gingerbread to Honeycomb(which was primarily for tablets) was a huge design change as it saw Holo design being introduced. Similarly from Honeycomb to ICS with the introduction of the notification toggles and change in iconography. Kitkat to Lollipop is most noticeable, with Google switching over to their material design philosophy.
• A.P.x to A.Q.x —-> Indicates a minor changes with some new features.
  A good example of this is the switch from Jellybean to KitKat when Google ditched the Holo Blue color adopted during the transition to Gingerbread for new White accents in the user interface.
• A.P.X to A.P.Y —> Indicates minor bugfixes to the last released Android version. A good example to this is the power leak and stutter in some older Nexus devices in Lollipop 5.0 was addressed in the version 5.0.1 and 5.0.2