pressure treated wood is wood treated to withstand more pressure. pressure treated is good for roofing cause of snow/ice, whereas solid wood should be fine for walls. i also recommend pressure treated for most suspended flooring.
Pressure treated wood refers to the fact that standard wood is put in a vat of special chemicals to retard insects and spoilage and put under pressure to ensure that the fluids flow throughout the wood. The wood is structurally no different than other wood, it just won't rot as fast, and insects will leave it alone. If you are building outdoors, most building codes require pressure treated wood to be used.