Short answer
A projector's Throw Ratio dictates how far back the projector must sit from the screen to produce a given image width.
$$\text{Throw Ratio} = \frac{\text{Distance from Lens to Screen}}{\text{Width of Screen}}$$
[Standard Throw] ===============================> Screen (10 - 15 ft away)
[Short Throw] =======> Screen (3 - 8 ft away)
[Ultra Short Throw] =| Screen (Inches away)
The Three Main Throw Categories
Standard Throw (1.2:1 to 2.0:1): Requires approximately 10 to 15 feet of distance to cast a 100-inch image. Best for rear ceiling mounts or back-of-room placement.
Short Throw (0.4:1 to 0.9:1): Utilizes specialized wide-angle lenses to cast a 100-inch picture from just 3 to 8 feet away. Ideal for small apartments, golf simulators, and setups where people might walk behind the projector and block the beam.
Ultra Short Throw / UST (0.15:1 to 0.3:1): Uses complex wide-angle mirrors and inverted optics to sit directly against the wall, projecting a 100-inch to 120-inch screen from mere inches away on a media console.