Varieties of hardwood lumber are the staple materials for the woodworker, particularly those who focus on fine woodworking projects such as furniture. However, the term hardwood can be a bit deceiving, as it has less to do with the "hardness" of the material than the species of the tree from which the lumber is harvested.
Hardwoods come from deciduous, or broad-leaved trees, as opposed to softwoods, that are harvested from from evergreens.
In general, the lumber derived from hardwood species are typically harder than softwoods, although there are exceptions (balsa wood is very light and soft, but is considered a hardwood). Most hardwood tree species lose their leaves in winter, and generally offer a much wider variety of colors and textures than softwoods. Typically, stock from hardwood species are a lot more expensive than those from softwoods.
Which Hardwood to Choose?
When preparing to build a project, the choice of which hardwood material to use can be a daunting question. To make it easier, start with determining how you want to finish the project. Will you stain or paint it?
If you choose paint for your finish, you won't want to waste your money on woods known for their color and beauty when stained, so avoid richly-colored species such as oak, maple, walnut or mahogany. For painted projects, poplar is a very good choice because it is relatively stable and takes paint quite well (not to mention that it doesn't look all that good stained).
However, if you want to stain or clear-coat the project, you'll have a number of choices to look through. Since your local home center megamart will probably only carry a couple of hardwood species (poplar and red oak are common) spend time at a fine wood supplier and look through the vareties available.
They should be able to help you determine how each species will look when finished, which will go a long way toward refining your decision.
Location, Location, Location:
In addition to the type of finish you want, the location of the final installation should be considered when choosing a hardwood species. While it won't have as much bearing on furniture pieces to be used indoors, you may want to consider some more moisture-resistant species (such as cypress or the ever-increasingly endangered teak) for outdoor projects. Again, your local woodworking supplier will be able to help with this decision if you are unsure about what species might work well for your particular application and climate.
One subject that a woodworker must always consider when building a project is how seasonal moisture and temperature fluctuations will cause expansion and contraction of the wood stock in the assembled project. For instance, if you've ever experienced a drawer that sticks only in the winter time, you've experienced seasonal movement of wood. Since each wood species is affected by these temperature and moisture fluctuations, you'll need to know a bit about the climate where the project will be used, and how your chosen wood stock is affected by the climate changes.
Again, your local woodworking supplier can be a great resource for answering these types of questions in your area.
If you'd like to learn more about the differences between hardwood species, I can think of no better resource than R. Bruce Hoadley's 1980 masterpiece, Understanding Wood: A Craftsman's Guide to Wood Technology. Not only does Hoadley detail nearly every species used for woodworking, he does an exceptional job at describing how to prepare, work with and finish these hardwoods. This is an industry-standard resource, one that I'd highly recommend to every woodworker.