Because wood is a naturally occurring material in the environment, it's subject to rot and decay if exposed to moisture, insects or vermin. Wood doesn’t last forever—unless it’s infused with chemicals to preserve it. This is where pressure treating comes into play. By immersing a piece of lumber (harvested wood) in a liquid preservative within a pressure chamber, chemicals are forced deep into the wood fibers.
Until recently, the chemical most commonly used in this process was chromated copper arsenate, which is considered quite hazardous to humans. (Arsenic, anyone?) Fortunately, more human-friendly, copper-based chemicals are used widely today.
There are varying levels of treatment, depending on the intended application for the wood. The details of the application should be posted with either ink or a plastic tag at the end of each piece of lumber.