Channel Proteins extend through the membrane and acts as a gate. This gate regulates what comes in and what leaves the cell. These proteins are specific in the materials that they will allow / transport through the membrane.
Receptor Proteins - Specific proteins relay messages from the environment to the interior of the cell. Only material with the correct shape fits on the end of one of these proteins. When a match is made, the receptor protein changes conformation (shape). This triggers a response from the inside of the cell.
Marker Proteins - The are short proteins that do not extend through the membrane completely. These will generally have a carbohydrate that sticks off of the protein to the environment that surrounds the cell. This will be used as tag for purposes of identification.