An A Frame is a basic structure designed to bear a load in a lightweight, economical manner. The simplist form of an A frame is two similarly sized materials laid up against one another, and attached at the top in a convenient fashion. These materials are often wooden or steel beams arranged in a 45 degree or greater angle. These materials are then lashed together with rope, or secured in another fashion such as welding, [glue]]ing, or riveting.
Due to having only two "legs", A frames are usually set up in rows so that they can have good stability. A saw horse is a good example of this structure. More complex structures will have a crossmember connecting the two materials in the middle to prevent the legs from bowing outwards under the strain of the weight.
Additional structures that use A frames: