16 pixels by 16 pixels is a common resolution for assets matching a low-res pixel art aesthetic. Although the assets may not be specifically 16x16, they are designed to work in a graphical setting where objects are around that size.
Typically refers to a style of game where a player's ship can rotate 360 degrees on a 2d plane and is responsible for shooting and destroying on coming asteroids. The asteroids start out large, but divide into smaller obstacles as they are damaged. The game becomes more difficult and chaotic as asteroids are broken into the smaller pieces.
