Barron's Accounting Handbook has the following definitions:

Capital stock:
Equity shares in a corporation authorized by its Articles of Incorporation and issued to stockholders. The two basic types of capital stock are COMMON STOCK and PREFERRED STOCK.

Stockholders' equity:
Ownership interest of stockholders in the corporation; also called Shareholders' Equity, OWNERS' EQUITY, and NET WORTH. Stockholders' equity is the difference between total assets less total liabilities. The components of the stockholders' equity section of the balance sheet are:
Plus: Retained earnings
Unappropriated
Appropriated
Plus or Minus: Foreign currency translation gain or loss
Minus: Unrealized loss on long-term investment portfolio
Total
Minus: Treasury Stock
Total Stockholders' Equity

Hope this helps.
In my 22nd year of translating almost exclusively for Mexico City based member firms of international big 4 and 2nd tier CPA firms, as well as other financial entities.

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Note added at 11 mins (2010-06-15 21:48:52 GMT)
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Capital stock and stockholders' equity are US English.