Having trouble understanding what tension is

Hi,

I undersand that tension is the force exerted on an object by a string or rope when pulled. Where I get confused is when I'm asked to find the tension at a point along a rope or tension in a section of rope...would the tension in/of a section of rope be the net force acting on it, or the force it exerts on either end (if this is true, I assume the force acting at each end is necessarily always the same?)?

Sorry if I'm not making myself clear...here's an example which might show what I mean...

A verticle rope is attatched at both ends (to walls). Find the tension T at distance y above the lower end of the rope. The mass of the section below y is M and the mass above is m.

Hope I make sense!

Thanks,
Matt