These two errors are confused by many! An Off center error is a Planchet that was struck partially out from under the dies. It will be missing some of the design of the intended coin. Example: it will be missing a portion of In God We Trust, Liberty, or other design. Only part of the intended design for a coin will be struck into the planchet. It can be from 5% to 95% Off Center, BUT a portion of the design MUST be missing.
A Broadstrike is a similar error, but is NOT missing any design. It will appear larger than a regular struck coin, because it was struck out of its collar. It can look Off center and still be a Broadstrike error, if NONE of the design of the coin is missing.