California's General Corporation Law refers to "common shares" rather than "common stock".  What makes shares shares "common shares"?  Section 159 defines "common shares" as "shares which have no preference over any other shares with respect to distribution of assets on liquidation or with respect to payment of dividends".   Note that under the statute, the fact that a class of shares has greater voting rights does not make it any less common than a class with lower or no voting rights.  If a corporation has only one class of shares authorized, they will necessarily be "common shares".

The GCL does not attach many consequences to classification of shares as "common shares", but it is important to know what they are in several situations.   For example, Section 402(c) prohibits a corporation from issuing or redeeming redeeming redeemable common shares unless the corporation at the time has outstanding a class of common shares that are not subject to redemption.  There are three exceptions to this prohibition under the statute pertaining to specific situations.  

What if shares do not meet the definition of "common shares"?  Such shares are by apophasis "preferred shares".  Cal. Corp. Code § 176.