What's the difference between a fender and a quarter panel?  Unless you are a car oriented person, you may not know if there is a difference between the two.

Similarly, many people use the words "speed" and "velocity" interchangeably, even though there is a difference in their meanings.

SPEED is merely a number that gives a magnitude (magnitude= size or amount)
For example, when you say, "We are going 100Km/Hr", this only tells someone the rate you are moving at.  They know nothing else.
Since speed represents only a magnitude, it is called a SCALAR quantity.

VELOCITY is SPEED+DIRECTION.  If you tell someone that you are going 100Km/Hr due North, then this is a velocity.
Since VELOCITY is a magnitude+direction, it is called a VECTOR quantity.

Mathematically speaking, V=DX/t

This means that velocity can be positive, negative, or even zero.  It depends on your point of view... If you are walking to the back of a moving bus, what is your velocity?  It depends on who you ask... You think you are going at your walking speed in one direction BUT someone on the sidewalk watching you would say you are moving fast in the opposite direction.
Login to Lecture OnLine and answer do the problem called, "common velocity".

Go to this site and see what happens to the velocities of the objects, depending on your point of view.  Wherever you put the cursor, you can see the vectors for the other objects in/near the river.  I suggest you click the "info" box.  Then, right click anywhere and then left click&drag on the person's leg==>then he will move into the river...READ THE DIRECTIONS!