Shares authorized, issued and outstanding refer to the total number of shares a company is legally allowed to sell (authorized), has actually sold (issued), and are currently held by outside investors (outstanding).
For example, a company's articles of incorporation authorize 10,000,000 shares. It has issued 2,000,000 to founders and early investors, of which 1,800,000 are outstanding (the remaining 200,000 having been bought back). Investors use outstanding shares to calculate current ownership percentages.