Speed only tells you how fast something moves. Velocity is a vector quantity, so it has a size (the speed) and a direction. If either one changes, the velocity changes, and the object is accelerating.
A planet in a circular orbit moves at constant speed. The force of gravity from the Sun always pulls it towards the centre of the circle, which is at right angles to the direction the planet is moving. A force at right angles to the motion cannot speed the planet up or slow it down. It only changes the direction of travel.
So the speed stays the same but the direction keeps changing, which means the velocity is changing. The planet is accelerating towards the centre of its orbit even though it never speeds up. The same is true of a moon or satellite in a circular orbit.
