Glance over this page and make sure you're familiar with all the terms so you don't get lost in the comments while programming.

sprite

An object in the game with a position in the world, eg a planet, a ship, a missile but not an OK button, or a status bar

world vs. screen coordinates

The world coordinates are like the Cartesian coordinates from math, it is the universal relativity used to identify the location of all sprites. The screen coordinates are then the actually location on the screen, in pixels, of a sprite. Sprites off-screen would then not have screen coordinates, or the coordinates would be negative or too great to be found on the display. Screen coordinates are usually only needed for erasing and drawing the screen, while world coordinates are used for updating sprite positions, performing game logic, and so on.

simulation

A scenario or the main game itself, a container for the game loop. The game itself is the simulation, with more specific functions, such as taking input and handling video concerns, are handled by classes such as InputClass and VideoClass.