What are epithelial tissues? What are their general function and how is that function associated to the features of the tissue?

Epithelial tissues, also known as epithelia, are tissues specialized in the covering of internal and external surfaces of the body.

The general function of the epithelium is to give protection and impermeability (or selective permeability) to the covered structure. This justifies the epithelium's typical features: the cellular juxtaposition forming layers of very nearby cells with diminished or none intercellular space between each two neighbor cells.

Cell Tissue - Image Diversity: epithelial cell