These are the largest, most well known companies with market capitalization over $5 billion. Companies such as: Wal-Mart, Microsoft, General Electric, Apple, Exxon Mobil. Most of these old established companies are one the New York Stock Exchange (see Stock Markets & Security Exchanges).
Medium Cap (Or Mid Cap)
This is the next step down in size, those between $1-$5 billion. You probably are familiar with many of these stocks as well. Most have been established for a while, although not as long as the large cap stocks: Quicksilver Resources, Netflicks, Aeropostale, Priceline.com., are a few of the Mid-Cap stocks.