Velocity:

It is common to find people confusing the terms velocity and speed when describing how fast an object is moving.  The difference is that speed is a scalar quantity, whilst the term velocity refers to both speed and direction of an object.  The full definition of velocity is that it is the rate at which its position changes, over time, and the direction of the change.

The simple diagram below shows how an aircraft, which flies the irregular path from 'A' to 'B' in an hour, (a speed of 350 mph), has an actual velocity of 200 mph in an East-Northeast direction.

Acceleration:

This term describes the rate at which velocity changes.  If an object increases in speed, it has positive acceleration; if it decreases in speed, it has negative acceleration.  A reference to Newton's Second law of Motion will explain the principles of acceleration.  Acceleration can be in a straight line, which is referred to a linear acceleration and it can apply to rotating objects whose speed of rotation is increasing, (or decreasing), when it is called angular acceleration.