use this for an example...

you get in your car and start roaming the streets...
the scenery flashes by your window... the faster you go,
the scenery goes faster...
you go around the block... soon the scenery becomes the
same as before...
a dog crosses your path (sprite)... you beep your horn...
using the curbs as your boundry so's not to hit a building...
unless you want that is... haha!
etc... etc...

everything pretty much becomes your engine... you program
different subroutines to cover, scenery, movement, sprites,
which all become invisible to you as you drive... they are just
there running... the variables would be pretty much your
location, and how fast or slow you drive... oh yeah! and the
dog's...