In its simplest presentation, a baseboard is a simple piece of wood skirting that covers where the wall meets the floor.
However, it can be as decorative as you like, acting in a functional way as well. Baseboards have been installed at various depths called skirting boards, mop boards (preventing walls from damage while mopping he floor) floor molding and base molding. In many ways, baseboards have protected the lower parts of walls from any damage cause from a stray foot kicking or other bumping that could cause an abrasion or unsightly scar or marring. Baseboard can also prevent furniture from smacking up against the wall when moving it about.
Baseboard design can pretty well be left up to imagination. Although a trend in modern architecture is to limit the baseboard molding to a simple design, older homes display baseboards comprised of sometime two, if not more, molding types to create a more decorative look. A favorite use is to apply quarter-round molding to the top of either flat or beveled wood molding.
Although baseboard molding can be created from almost any type of wood, soft woods, like pine, are the preferred choice for its ease in handling and cutting.