What do you mean by using EV as in exposure? Are you talking about EV as a unit of measurement, or are you actually referring to exposure compensation? EV stands for exposure value - 0EV means with a given set of exposure settings, you are getting an exposure of 18% grey at the metered spot.
Most (if not all) light meters register a well-exposed image as being 18% grey. Assuming you are metering a white wall on Program AE Mode with no exposure compensation, the camera will give you a set of settings to shoot with, correct? Let's say you take that set of exposure settings and shoot the photo, your wall in the picture will not look white, but grey (i.e. underexposed). Similarly, shoot a black wall with no exposure compensation on Program as well; your black wall will look grey (i.e. overexposed)
Hence, since camera meters push white down to grey, and black up to grey, one has to be able to know when to override these automatically generated settings to render white as white (overexposing by about one stop), and black as black (underexposing by one stop).
In terms of tricky lighting situations, one has to look at what is the intended subject that needs to be well-exposed. Sometimes, you have to compromise by blowing out a certain amount of highlights to ensure your subject is well-exposed; exposing for the highlights may mean your subject is underexposed. Makes sense?
