There is usually some confusions between speed, velocity and acceleration when people are new to physics. But actually these terms are not difficult to differentiate. First of all, speed is a scalar quantity while velocity and acceleration are vector quantity. Scalar quantity refers to quantiry which has no direction but only magnitude, while all vector quantities have both direction and magnitude. Speed is the magnitude of velocity, and it refers to the distance an object travel at a specific amount of time. So its unit is distance/time (the standard unit is m/s, meter per second). Velocity uses the same unit, but veloctiy is calculated by displacement dividedby time. What is more, veloctiy can be either positive or negative, while speed can not. This is because veloctiy is the rate at which an object changes position. If you walk from here to there (doesn't matter where), and go back here from there (exactly at the same spot) when 5 minutes has passed, then in 5 minutes your velocity is zero because there is no change on your displacement. The positive or negative sign for velocity is for representing directions. For example, when a man throws a pingpong ball upwards, the velocity carries a postive sign, but when the pingpong ball falls back, the velocity at that time carries a negative sign.                        Acceleration is totally different from veloctiy. Acceleration tells you how fast the velocity changes. So, if you get a velocity of zero, if  doesn't mean the acceleration is zero; when velocity is very high, cceleration doesn't have to be high as well, and it can be zero or negative too!! Let's use the pingpong ball case again. Actually that kind of motion is called projectile motion in physics. When the pingpong ball reaches the maximum height it can reach, it will start falling immediately right? So at the moment when it reaches the maximum height and before it starts falling, it has a velocity of zero, but its acceleration is not zero there!!!!! Whenever an object is changing velocity, then it is accelerating. Guess what is the unit of acceleration? Well, since acceleration is changing rate of velocity. it is velocity (m/s) devidedby time (we use seconds here as well to make things easier to understand). so "m/s" devided by "s" is m/s2.                                                                                                                                                                                    There is one more thing to know. In calculus, acceleration is the derivative of velocity.