What is the market cap of a stock on the stock market and how does it effect me / my investment?

Total shares outstanding * share price = market capitalization

Large-cap stocks are more liquid (for example, loses less transaction costs) and tend to be more stable (for example, that sleep better at night) with a more institutional. (leading to stability).

How it affects you.

Nothing that affects unless you buy it. With that said, there are index funds and actively managed mutual funds that buy stocks based on their market capitalization. The market capitalization of a company, you care whether you're buying a fund that seeks to invest in it. In our example above, the Corporation An example is a small-cap stocks. So if you are buying a small cap fund, then it would mean something to you.