What is it?  A rehearsal dinner is typically an informal meal after a wedding rehearsal, which takes place the night before the actual wedding.  It's a chance for the wedding party to get to know each other better, and most importantly, it's a chance for the bride and groom to thank all the people who have helped them out with the wedding planning.

Who pays?  Traditionally the groom's parents pay but nowadays you'll often see couples include the cost in their overall wedding planning budget.  Another scenario is for both parents to host it together.

Who plans it?  It is traditionally the realm of the groom's mother. Check in with your mother-in-law and see if she is willing and able to do so.  If she wants to, it's a good idea to let her – it gives her a role in the wedding and even if the rehearsal dinner is completely different from your taste, guests will presume that you didn't plan it.

 The familiar tradition of a flower girl throwing rose petals as she passes down the aisle before the bride is a reminder of days gone by when the bride walked to the church with her maids in waiting.  Leading the procession was always a young girl throwing flower petals along the lane, so the bride's path through life would be happy and laden with flowers.