And there's a wide variety for you to choose from. If you're planning outdoor construction, use pressure-treated lumber, which resists rot and may be placed in direct contact with the ground.
Treated lumber is available in standard construction grade and with a water-resistant coating.
A grading system indicates the properties and quality of a piece of lumber. For example, grade .40 can make ground contact without rotting and is good for deck construction. Number-1 lumber is better quality, with fewer knots and blemishes. It's okay for wall studs to have visible blemishes and knots because they'll be hidden behind the dry wall, but make sure the lumber is straight. Number-2 pine is harder than the wood used for wall studs and is more difficult to drive screws and nails into.