Here is a brief explanation on Common Shares.
Advantages of owning common shares:
1) A marketable asset
2) Potential for capital appreciation
3) Favourable tax treatment
a) Dividends
b) Capital gains
4) Voting - a say in the affairs of the company
5) Dividend income
6) Limited liability investment
Most suitable for long-term investment (10+ years); hopefully your company doesn't go under 
Problems: can be very volatile (risky) over short periods of time.