Speed tells you how fast something moves, and it has magnitude only, so it is a scalar. Velocity is speed in a stated direction, so it is a vector. Both are measured in metres per second, m/s.
A cyclist moving at 6 m/s east has a speed of 6 m/s and a velocity of 6 m/s east. A velocity of 6 m/s west has the same speed but is a different velocity, because the direction is different.
This means velocity can change even when speed does not. A car driving round a bend at a constant 15 m/s keeps the same speed, but its direction is changing, so its velocity changes. A train going 30 m/s north and then 30 m/s south has the same speed in both cases but a different velocity.