A FORCE IS A PUSH OR A PULL
We always teach that a force is a push or pull, but thanks to Howard for pointing out that a force is actually only a PUSH and it must be in the direction of a the force.
For example if a car is towing a caravan, we think of it as pulling the caravan, but in reality it is the front of the tow bar which is pushing forwards on the connector to make it move forwards.
if you pull a book along the table, the actual only forces are those from your fingers forwards on the contact point of the book - think what would happen if the book was made of soft putty instead of paper.
I do wish to add however that this is really only for 2 bodies interacting - ie OBJECTS pushing or "pulling" each other. We have to consider forces as "pulls" in situations like "opposite electric charges attracting each other" and "objects attracted to each other under gravity" (Thanks for Michael for drawing my attention to this)