There are a number of differences between institutional investors and non-institutional investors. If you are considering an investment in a particular stock that you've seen publicized in the financial press, there's a good chance you don't qualify as an institutional investor. In fact, if you're wondering what an institutional investor is, you're probably not an institutional investor.
Institutional investors are the big guys on the block - the elephants. They're the pension funds, mutual funds, money managers, insurance companies, investment banks, commercial trusts, endowment funds, hedge funds, and some hedge fund investors. Institutional investors account for half of the volume of trades on the New York Stock Exchange. They move large blocks of shares and have tremendous influence on the stock market's movements. Because they're considered to be knowledgeable and, therefore, less likely to make uneducated investments, institutional investors are subject to few of the protective regulations that the Securities and Exchange Commission provides to your average, everyday investor. (See Policing The Securities Market: An Overview Of The SEC.)
The money that institutional investors use isn't actually money that the institutions have raised themselves. Institutional investors generally invest for other people. If you have a pension plan at work, a mutual fund or insurance, then you are actually benefiting from the expertise of institutional investors.
Non-institutional investors are, by definition, any investors that aren't institutional. That's pretty much everyone who buys and sells debt, equity or other investments through a broker, bank, real estate agent and so on. These are the people or organizations that manage their own money, usually to plan for retirement or to save for a large purchase.
To find out more, read Institutional Investors And Fundamentals: What's The Link?