What is the difference between speed and velocity?

In expansion to the above responses (and just to make
clarification), Speed is known as a Scalar and is dependant only on Magnitude (in m/s
using SI units, not m/s^2, that is accelaration, its different), but doesn't necessarily
use symbol (a) because (a) is also used as universal constant for acceleration. Speed
can be determined by the equation S=d/t; i.e, Speed equals the distance over
time.

Velocity on the other hand is a Vector which relies
on both Magnitude and direction.
As an example to differentiate the two, speed
would be 100 km/h, whilst velocity would be 100 km/h North.
The direction in
velocity is directly linked to displacement, or how far from where you originally
started. So, if you were to travel 50 kilometres one way (east) your displacement would
be 50 kilometers East, but if you were to travel 50 kilometres one way and 50 kilometers
back, your displacement would be 0 kilometres because you would be back in your original
position (this is important in relative motion)
Velocity is usually measured
in m/s with direction, but can also use m/s^2 when using velocity to determine
acceleration because Acceration (a)= [final velocity in m/s(Vf) - initial velocity in
m/s (Vi)] / change in time in seconds(Delta t). After substituting, you get an
acceleration in m/s^2.

Make sense? Message if you have any
more queries.