Steak comes in various shapes and forms, but is usually a block of meat - most often beef - which is cooked*, and sometimes served with vegetables and chips.
The extent to which the steak is cooked is primarily chosen by the person who will be consuming it:
Blue - very lightly cooked, very bloody on the inside.
Rare - lightly cooked, with only the outer layer turning brown. Mostly red inside.
Medium rare - lightly cooked, with a slightly deeper cooked outer layer. Red inside.
Medium - cooked through, with deep cooked layer. Slightly red inside.
Well done - cooked through thoroughly, no red inside.
Thoroughly cooking steak is safer, and is less likely to give you food poisoning... but the rarer the meat, the juicier and tastier it is.