Speed tells you how fast an object is moving. It is a scalar quantity, so it has size only. Velocity is speed in a given direction, so it is a vector quantity with both size and direction. This means velocity changes if either the speed or the direction changes.
An object moving in a circle at constant speed is always changing direction. At any point on the circle its velocity points along the tangent, a straight line touching the circle at that point, and the tangent points a different way at every point. So the direction of the velocity keeps changing, even though the speed stays the same.
A change in velocity is an acceleration. An object in a circular orbit at constant speed is therefore accelerating all the time. Examples are the Moon going round the Earth, a satellite in orbit, and a car going round a roundabout at a steady speed. In each case the speed is constant but the velocity is changing.
