Organs are composed of tissues, which are in turn composed of cells. Functions include lining, protecting, and forming glands. Cuboidal epithelium is cube-shaped cells. Columnar epithelium consists of elongated cells. Any epithelium can be simple or stratified.
Simple epithelium has only a single cell layer.
Stratified epithelium has more than one layer of cells. Pseudostratified epithelium is a single layer of cells so shaped that they appear at first glance to form two layers. Note the single layer of simple cuboidal epithelium lining either side of a tubule. I have outlined one of the columnar epithelium cells. Secretion of a product.