Android is an open source operating system avalibe for mobile devices, TV and even computers .Android OS is one of the most widely used OS in the world. Anyone can develop app and it be uploaded on the app store like play store ,amazon play store ect .Most mobile devices will accepts app form these kind app stores .
And most device won’t allow you to install apps that are not available in approved mobile app stores.
Installing 3rd party apps:
There are two ways to install 3rd party apps .These 3rd party app are available in site like mobiginie,99apps..ect.Lets see how to install those apps. One by changing your mobile settings to allow to install 3rd party apps .This option can be found on Devices >advanced option/ setting.and the other is a bit complex you have root your mobile by process called Mobile routing .you can install apps by this method. If you are going to install these kind of 3rd party app beaware some app might steal your data or it can be an adware.