Trees are usually the most important, and they're always the most permanent aspects of your landscaping. Because of their size, they provide a focal point around which the rest of your landscaping develops. In addition, they work wonders for both the environment and the aesthetics of your home.
Your first choice when it comes to trees is one of size, and remember to select trees based on their eventual, not current, size. Small trees include the amur maple and Japanese maple while birch trees are a good example of medium-sized trees. Maples, oaks, and sycamores, are all large shade trees. Evergreen trees (e.g. pines and spruces) are also often large, but they do not provide much shade for surrounding plants.
In addition to size, however, you should think about what trees will flourish in your area. If you suffer from regular storms, choose a sturdy tree while drought-prone areas call for water-conserving trees such as hackberries, white firs and pines, and red maples.
Finally, consider the growth rate of your trees. You should plant trees regardless of their growth rate, but unless you're landscaping for your grandchildren, don't plan an entire landscape around a tree that won't be tall for two generations.