You said, "When you're looking at pruning for shape, for example, is the idea that you want to basically prune off anything that isn't going to contribute to the shape you want?"
The basic answer is yes.* Pruning is a bit of an art as well as a science.
You said, "I've got a maple tree, red maple,... but the shape of it is more or less like a pipe - cleaner!* It is growing up almost as much as it is growing out."
The reason for this type of growth pattern is most likely the fact that it was grown in crowded conditions in the nursery.* That leads to trees that look like this.
You said, "Now, if I go in and, for example, prune off of the lower branches any growth that is "on the top", i.e., growing up rather than out, will that tend to influence those branches to grow more horizontally?"
Unfortunately the answer is no.
This site is especially good for info on pruning young trees.
Some great info here, especially the pics showing where and how to prune.
This might be a bit too much info here, but if you scroll through this site it will give you an idea of how trees should be grown in the nursery and what to look for.* At the first link on the page for 'Selecting quality trees from the nursery', do especially read through their page 45 and on about trunk and branch structure.* It's a power point presentation.
Lots to read here, especially at the bottom of this first page and the links on the left under 'Trees'.
Hope that helps,
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.