What will happen if a defendant fails to pay an agreed upon judgement amount to a plaintiff in small claims courts?

The judgment creditor will avail himself of various legal mechanisms to collect, including garnishing wages, restraining bank accounts and other assets, etc.  Interest will run on the judgment until it is paid, and other expenses of collection (for example a marshal's or sheriff's fee if one of them has to get involved in collectin efforts) may also be added to the judgment.