Is a term which is widely used and perhaps the least understood by novices. It basically refers to large, stable stocks that are well known, have had great earnings in the past, and will continue to do so in the future. For example, IBM is considered a bellwether stock in the computer industry. Often times, the performance of a bellwether is an indication of how that industry is doing as a whole.
Market cap is simply the number of shares outstanding x the price of the stock.