A torus is a donut. Take a circle, then sweep its center around a bigger circle — what you trace out is a torus. It's the simplest shape in 3D space that has a hole going through it.
Volume
V = 2 π² R r²
R = distance from torus center to tube center; r = tube radius.
Surface area
A = 4 π² R r
Topologically, a coffee mug and a donut are the same shape — both have exactly one hole. That's the famous 'topologists can't tell their breakfast apart' joke.