The American Heritage Dictionary defines a tree line as:

1. The limit of northern or southern latitude beyond which trees do not grow except as stunted forms.
2. A timberline.

I find this definition to be a little lacking. First of all, AFAIK, the tree line has just as much to do with altitude as latitude. Without heading north or south, you can pass back and forth through the tree line, if you go up and down in elevation. I think a more complete definition would also mention this fact. Basically, the tree line is an imaginary line that separates areas that readily support the growth of trees from adjacent areas that do not. It is very easy to spot tree lines on tall mountains, because they look "bald" on top.