A creature's SPEED indicates the number of five-foot increments (squares) it can move in one action. SPEED is also its speed in miles per hour.
SPEED is the total of the raw dice pool of STRENGTH and AGILITY plus one relevant skill. Movement skills include (but are not limited to) running, climbing, swimming, flying, and zero-g.
Movement methods. For non-primary movement methods, the movement rate is halved. For most characters, this means that walking/running is at the full rate, and climbing, swimming, zero-g, etc. are at half rate (after adding the bonus from the skill). Some exploits may grant a full movement rate. Some creatures (such as birds) may have a different primary movement mode, and walk/run at half speed. A skill cannot turn a movement method into a primary method; only an exploit can do that.
Hustling. A character who moves with two actions in a turn is hustling, and effectively moving at twice its normal speed. Creatures with more than two actions available may not move more than twice in a turn.
Grids. If no grid is being used, the creature can move a number of inches on the tabletop equal to its SPEED. If a square grid is being used, a diagonal movement counts as 5', but creatures may not move diagonally around a corner.
Difficult terrain. Difficult terrain (deep snow, swamp, ice, high or low gravity, etc.) halves ground movement rates. Some exploits may allow full movement rates on one or more difficult terrain types.
Actions. If a character moves and takes an action in her turn, the action can take place at any point during that movement.
Long-distance. Long distance movement takes place at a character's SPEED in mph. Hustling (moving with both actions in a turn) doubles this movement rate.
Vehicles. A vehicle's SPEED its its speed in tens of miles per hour. A vehicle moving at 4 (40mph) moves at ten times the speed of a human moving at 4 (4mph).