The external faces of some of the proteins in the plasma membrane have carbohydrate groups attached to them

Proteins that have sugars attached to them are known as "glycoproteins". OFten, glycoproteins are involved in cell recognition. For instance, immune cells like T cells use glycoproteins on the surface of a cell to determine if the cell is foreign or if it is one of your own. Much of the Major Histocompatability Complex on the surface of a cell is made up of glycoproteins. In this sense, the glycoproteins are like the "name tags" of the cell.

Glycoproteins can also function in linking neighboring cells as part of cell junctions or as receptors for extracellular signalling molecules.

For a diagram of glycoprotein synthesis, see http://www.bio-cranial.com/images/glycoprotein.jpg