This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
A body is in tension when a force, acting on it parallel to its axis, tends to separate its particles by drawing them apart". A compression force is one that acts parallel to the axis of the body and tends to force the particles into one another. In short, a body in tension has a pulling force upon it, while, if in compression, a push would be exerted on it. A strain was at one time considered as a force acting on a body, but the more modern idea is to consider it as the change of form in a body due to the application of a force.