Speed is a scalar quantity (that is, without associated direction) which measures the rate of change of distance traveled with time.
Velocity, on the other hand, is a vector quantity which includes the direction of travel.
So, if an object is moving at a constant speed but changes direction, because of an accelerating force in a direction different to the direction of movement, the velocity changes but the speed doesn't. The moon travels around the earth at approximately constant speed but continuously varying velocity.
speed is the distance a moving object travels in a certain amount of time. The speed of an object is the ratio of distance to time, so to find the speed, divide the distance traveled by the time. make sure you label the speed with a unit of meters per second, or some other unit ratio like that.