This is a one piece architrave is also called a pediment or a door header. It is the top horizontal trim piece over the door or window. It will be wider and thicker than the casing. You do not mitre the casing around the door when you use architrave. The casing comes up each side of the door and butts into the bottom of the architrave. Architrave moulding is a very classic look and can come as a one piece mould, or often built up out of three or more separate trim pieces.
This cap moulding is more often used as a backband for 5/8″ thick builder colonial casing. A cap mould is a generic term for a moulding that is designed to cap something off. Examples would be a wainscoting cap mould and a header cap mould. A header cap mould is similar to an architrave only you mitre the casing on all three sides of the door, then add the cap on the top portion only to get a more neo-classic look.
This 2-Step baseboard is often used for other decorative purposes in millwork projects as well as being used as a baseboard. Baseboard ties the wall and floor together offering a visual foundation. It protects the walls from bumps and cleaning. The baseboard should compliment the style of casing and should be thinner and wider than the casing. Shoe mould often accompanies the baseboard if the floor is uneven or there is gaps between the baseboard and the flooring.