Softwood and hardwood typify the wood market. Hardwoods are graded and used aesthetically. Softwoods are typically graded and applied structurally. They're interchangeable for some applications, but neither is suitable for everything.
Soft and Hard
Hardwoods are generally stronger than softwoods, but softwood is used more often structurally because its more affordable than hardwood. There's no reason why hardwood can't be used structurally, except that it's not milled into structural lumber, and doing so would be costly.
Fir, pine, spruce and hemlock are the main sources of structural lumber. Framing lumber is the main go-to lumber for framing and building. The three woods are often a combination of hemlock, spruce and Douglas fir, and sometimes marketed generically as Hem-fir, hem-fir-spruce, or even spruce-pine-fir. When lumber is grouped like this, it's been graded for structural performance. It's a common practice and there's no worry about mixing it up.
Douglas fir is rated as one of the strongest of the softwood varieties. It actually rivals some of the hardwoods for strength. It's the best all-around lumber for studs and beams. May be more expensive than other varieties when singled out. It can also be stained and finished as trim, but lacks distinct grain patterns, and is considered somewhat bland.
Hemlock is straight-grained and light, but typically not used as often as Douglas fir. It's used more often for moldings and trim because it resists warping and twisting, and when stained, reflects more pronounced grain patterns than other softwoods. Hemlock has only moderate resistance to scratching and dents.
Pine is the not as strong as fir or hemlock. Pine has more knots, but it's more affordable that hemlock or fir. Pine has the advantage of aesthetics if you like rustic lumber for paneling, cabinets or furniture. It's only real disadvantage is that it's one of the weakest woods out there, and can be prone to warping and twisting.
Redwood, cedar and cypress are three softwoods with natural weather-resistance. They share structural properties with fir, but are more appropriate for decking, trim, and all exterior applications when aesthetics and weathering are factored in.
Redwood could be considered the gold standard for outdoor application. It's got everything: beauty, strength and it lasts a lifetime. Use it for decking, stair and bannister railings, decorative borders or anything else when only the best will do. Redwood has two major disadvantages: it's very expensive -- and it's listed as an endangered species.
Cedar shares weather-resistant qualities with redwood, but at a fraction of the cost. It's lightweight and strong. Use it for all kinds of exterior trim, fences, shingles and siding. Drawbacks include it's softness -- it dents and scratches very easily.
Cypress grows in swamps. The water repellent nature of cypress make it perfect for docks and exterior structures when moisture is present. It's strong, straight and priced mid-range for construction lumber. It can also be used as interior trim. Use cypress in Southern states as an alternative to other, sometimes hard to find, Western species, such as fir or redwood.
Oak, maple, walnut, cherry, birch and alder are commonly compared domestic hardwoods used in fine furniture and cabinets.
Oak has the tradition of strength and beauty. It's broad, flame-like patterns are easily recognized. It's priced mid-range for a hardwood, and available everywhere. Drawbacks are few, but oak can be hard to work with, and shatters and splinters easily if it's not cut with sharp, carbide-tipped blades. Use it for cabinets, furniture, doors and interior trim.
Maple is near the top of the list for hardness. It lacks the grain patterns of oak, but some prefer the creamy texture of maple. It's heavy and requires sharp, carbide-tipped blades to cut. It's typically more expensive than oak. Common uses include flooring, cabinets and trim.
Cherry and Walnut
Cherry and walnut are two domestic hardwoods that are often grouped with exotic hardwoods because they share a deep, natural beauty unmatched by the others. Cherry is reddish, with fine, complex grain patterns. Walnut has a deep chocolate color. Both are some of the most expensive of all the hardwood varieties, with walnut at the top of the list. Use them for any application to display opulence and craftsmanship.
Birch and Alder
Alder is often used in conjunction with birch plywood to produce the most affordable cabinets on the market. The combination of alder solids and birch plywood provides the beauty of hardwood at a fraction of the cost. Alder lumber, when singled out, looks similar to cherry, with a light tan or reddish tint, and is sometimes called the "poor man's cherry." Alder is light weight, but dents and scratches with a fingernail.