A stock is a share of one unit of ownership in a company. So essentially if you own shares in a company you own part of the company.
As a shareholder you may have the right to vote for members of the board of directors. You may also be entitled to end of year profits based on the amount of shares that you own.
There are two different types of stock, common stock and preferred stock. Most stock that is held by individuals is common stock.
Common Stock
Common stock is the stock that is held by the public and normally represents the majority portion of the company. It can have voting rights plus a share of the dividends.
Preferred Stock
Preferred stock has fewer rights than common stock apart from dividends. In most cases when a company issues preferred stocks they pay consistent dividends year on year, plus if you have preferred stocks you get first call on dividends over common stock.
Investors would normally choose preferred stock because of its income potential from the dividends, so when a company produces larger profits they will share a portion of these profits with the shareholders via dividends.
Liquidity
A benefit of common stocks is that they are most of the time very liquid. Smaller companies may not trade with regular occurrence compared to larger companies who would normally trade stock daily, allowing you to buy and sell shares.