Moldings perform many functions as well as add architectural interest. 
Casing covers the unfinished edge between sheetrock and jambs around windows and doors.
Base provides a finished look at the floor line.
Chairrail is used to protect a wall from chair damage or to separate the upper and lower walls which may have different colors or textures.
Crown is used as an architectural accent, defining the ceiling line with an interesting shape, texture or color.