Just type that into google. I am sure you will get much more detailed information than what I am going to give you here.
If I remember the specifics right, a cash cow is a product that has a high market share and low growth. Meaning many people own the product but the product sales is no longer growing at an exponential rate. The cash cow is generally a prized product to have because it takes minimal marketing and pr advertising to promote it, no significant research and development expenditures have to be made towards it, and it is a fairly predictable source of revenue.
An example of this would be Proctor and Gamble's Crest Toothpaste. Crest has a large share of the toothpaste market, but I would doubt Crest's toothpaste sales increase much year to year. Although Proctor and Gamble may tweak the toothpaste a little by incorporating different favors etc, I can't imagine that they capture much more of the market or increase the growth in sales of it significantly. These relatively small expenditures towards making the toothpaste "better" are most likely to keep Crest relevant, or put in other terms, KEEP it a cash cow.