A mega cap refers to a company with a market cap of more than $200 billion or $250 billion.
The big cheese, the big kahuna, Monstro the Whale. These are the biggest of the big companies and where they tread, the earth trembles. "Mega" means "one million-times" or 10^6, but that's just too small for these bad boys.
At the end of 2008, there were only two companies in the S&P 500 index that qualified: ExxonMobil and Wal-Mart Stores. A year before that, six qualified at more than $200 billion: ExxonMobil, General Electric, Microsoft, AT&T, Procter & Gamble, and Google. So you can see that as share prices rise or fall (as happened in the late fall and early winter of 2008), companies can move into or out of top list.
For investors, there can still be value to be had by investing in these. They generally pay a dividend and it is possible for them to grow significantly. For instance, Wal-Mart's market cap increased 15% from the end of 2007 to the end of 2008. Of course, they can fall as well. Exxon dropped 20% over the same time period.
These also tend to be the most heavily followed companies by investors and Wall Street analysts, so the chance of finding some bit of knowledge to give you an edge that nobody else has found is remote, at best.