A company that does not have its shares listed on a stock exchange is an unlisted company.
An investor can buy shares for such a company from other shareholders or through a new issue.
There is no visible price which gives an idea about the value of the investment.
There is also a lower amount of liquidity in such shares as a buyer or seller needs to be found to transact.
Many investors buy shares in unlisted companies wanting to profit from it when these companies get listed on a stock exchange at some later date.