Yeast cell walls are also known as yeast hulls or yeast ghosts. Many sales reps sell yeast nutrients that they claim contain yeast cell walls when in fact they contain the whole yeast that has been inactivated. So what is the difference in application between cell walls and the whole inactivated yeast?
First a basic lesson in yeast cell morphology: yeast consists of a cell wall, on the inside of that you get the cell membrane and on the inside of that, well… the rest of the yeast. The cell wall consists mainly of glucans and mannoproteins (so-called polysaccharides) and the cell membrane consists of lipids (the fancy scientific name for fat). The lipids are made up of sterols and long chain fatty acids.
Both cell walls used on their own, and inactivated yeast containing the cell wall, membrane and yeast insides, stimulate fermentation. They do this in different ways though. Inactivated yeasts serve as a yeast nutrient in that they can be a source of vitamins and minerals. They can also be a source of sterols and long chain fatty acids. Live yeasts are little cannibals feasting on their dead counterparts when the going gets tough, i.e. the alcohol gets higher. To survive they need to strengthen their cell membranes and to do this they need more sterols and long chain fatty acids. They produce the latter in the presence of oxygen only. During fermentation oxygen is limited so they go for option B and that is to obtain these components from their dead mates. Reminds me of the movie Alive about the soccer team whose plane crash landed in the Andes Mountains. Yummy… Inactivated yeast insides also contain compounds such as amino acids and nucleotides that can “leak” out into the must and serve as a nutritious bite to the live cells.
Yeast cell walls are not a source of nutrients. They detoxify the must by removing medium chain fatty acids from the must, making the environment nicer and easier to ferment in for the live cells. Okay…long chain, medium chain, what’s the story? Live cells need to have an intact cell membrane containing long chain fatty acids to survive increasing alcohol toxicity. As alcohol levels become higher during fermentation, the live yeast starts to produce more fatty acids; however this process requires oxygen, which is limited or completely absent. The manufacturing of the fatty acid gets interrupted and the result is a much shorter chain that cannot be used in the cell membrane. These chains either stay associated with the yeast cell or get secreted into the medium. They physically block sugar uptake. For some reason they bind to yeast cell walls added to the must and the inhibition gets lifted.  
The commercial production of yeast cell walls is a much more complicated and expensive process than that of producing inactivated yeast. It is therefore usually a more expensive product. Depending on your specific fermentation conditions, you will use one or the other.
For a more comprehensive and slightly more serious explanation of the different types of yeast nutrients read the article, Wine yeast nutrients 101.