Authorized is the total number of shares a company can legally issue.

Issued is the total number of shares that have been sold, or in some other way placed in people's hands (compensation, exercise of warrants, ...).

Outstanding is the total number of shares that are currently in the public domain (owned by people, other corporations, investors, etc.).

Issued shares and outstanding shares are many times the same number.  But if a corporation buys back some of the issued shares, those shares are no longer outstanding.  The number of issued shares remains the same, since they were at one point issued, but the number of outstanding shares is reduced by the number of shares the company bought back, because they are no longer in the public domain.

Other factors can come into play, but that is basically how it works.