Post and lintel (also called an Architrave) is a simple construction technique, also called "post and beam", where a horizontal member (the lintel) is supported by two vertical posts at either end. This very simple form is commonly used to support windows and doors.
The biggest disadvantages to this type of construction is the limited weight that can be held up, and the small distances required between the posts. Roman developments of the arch allowed for much larger structures to be constructed.
There are two main forces acting upon the post and lintel: compression and tension. The two posts are under compression from the weight of the lintel (or beam) above. The underside of the lintel is under tension, while the topside is under compression.