Android

Android is an operating system developed by Google as an open-source project. It has been used primarily for smartphones and tablets, but was also modified in order to be used for TVs, cars, and wrist watches.

Advantages

Allows running multiple applications at the same time
Supports different types of hardware
Uses a mainstream development language - Java
Low cost
Customizable
Good security; applications do not have access to system resources unless they ask the user for those permissions before being installed

Disadvantages

The source code needs to be adapted for each device individually, which means that companies that produce smartphones may deliver version updates slowly for older devices, or not at all.