Market cap is roughly what it would cost to buy all of a company's shares at today's price. It does not count the company's debt. To value the whole business, including what is owed to lenders, use enterprise value, which is market cap plus debt minus cash.
Companies are grouped by market cap size. Micro-cap is under $300M, small-cap is $300M to $2B, mid-cap is $2B to $10B, large-cap is $10B to $200B, and mega-cap is over $200B. Small companies have tended to return more over the long run, but their prices swing harder and fewer analysts follow them.