Operating system question

[cmdr430]

Sorry, I don't even know how to search my question....

So, here it goes...

When I power on my phone and it is up and running,how does the phone decide which apps are to be running?

i.e.

If I boot up my phone one time, maps, playstore, facebook may be running (in the settings,apps,running)....

and other times, there may be different apps running,aside from the standard google/android operating system...

I hope I have worded my question well enough to be answered....

Thanks

Cameron