Simply put, the money pooled in by a large number of investors is what makes up a Mutual Fund. This money is then managed by a professional Fund Manager, who uses his investment management skills to invest it in various financial instruments.
As an investor you own units, which basically represent the portion of the fund that you hold, based on the amount invested by you. Therefore, an investor can also be known as a unit holder. The increase in value of the investments along with other incomes earned from it is then passed on to the investors / unit holders in proportion with the number of units owned after deducting applicable expenses, load and taxes.