What's the difference between speed and velocity?

I got a rubber and tied it to the end of a piece of string. I held it over the middle of a dinner plate that I had put on the floor.

 Then I started to spin the the string in my hand gently so that the rubber went round in a circle, tracing out the edge of the dinner plate.

You really should do this if you can to see what happens. But if you can't find any string, take my word for it that the rubber goes round at at steady speed. It doesn't get faster and it doesn't get slower (though it will eventually run out of energy after a while).
Now I'm going to tell you that the rubber is ACCELERATING. Think about it: I just said that the experiment shows that the rubber doesn't get faster and doesn't get slower. It stays at a steady speed. But I said it was accelerating. Accelerating usually means getting faster. Now we need to remember VECTORS and SCALARS. Speed is a scalar; it is just a size (magnitude). But velocity is a vector; it has both size and direction. The rubber might not be changing speed (size) but by going round in a circle, it is changing direction. So speed stays the same but velocity changes. Acceleration is a vector too. It can mean a change in size (speed increases) or it can mean a change in direction - or it could mean both at once.
The direction changes in this example because the string is constantly pulling the rubber round into a circle. The string provides a resultant force to make it accelerate by changing direction. If you cut the string, the the resultant force vanishes and the rubber stops changing direction. It flies off in a straight line until it hits the floor. Try it!