Acceleration is an increase in velocity over time. Velocity is a measure of both, the speed of an object and its direction. Even if an object is traveling at a constant speed it can be accelerating if it is changing direction. Accelerating objects can be represented with distance-versus-time graphs. The slope of the graph is increasing because as time progresses the object is moving faster than the second before.
When a car accelerates from a stop light it increases its speed and the distance it is traveling each second.
Deceleration is when objects slow down.
A car decelerates when it slows to a stop at a red traffic light.