Stopping SQL Server Agent: Effect on runnings jobs
Yes, jobs are aborted. Non-SQL process (eg CmdExec) are aborted too.
No more jobs run until SQL Server Agent is started again. Missing scheduled jobs will not be run.


After comment:

SQL Server Agent is just another connection to the DB engine. It will send an abort to the DB engine and all SQL jobs are stopped.

Abort means something here: it is a specific client action. See SQL Server Transaction Timeout