Buttons usage:
Back Arrow is like the close button X on computer. If you're done using something, like camera, press Back to exit.
Home (the physical middle button) is like the minimize button _ on computer. Like when you're web surfing, and you want get back to it as you want to do something else, press Home. You then can hold Home, or press on its icon on homescreen, to re-open it.
I recommend you use Home button only when needed. Don't use it for everything, as putting apps in background would eat RAM.
In other words, don't treat it like a fruit-phone. Treat it more like a computer, like when you're done online shopping, you would close all tabs then press X to close the browser. And if you're running too many programs, doing too many things more than the computer can handle, it would start to lag and might freeze.
Also,
Menu (the left button) is like the menu bar on computer (the bar on top where it usually has File, Edit, View, etc.). The Menu button, when pressed, usually has things like Settings, Help, Info About App, etc.
And long-pressing on the screen is like right mouse click.