The wedding rehearsal is the dress rehearsal for the wedding ceremony. It's a chance for the officiant to explain to everyone in the bridal party what will happen during the ceremony and the bride and groom will finalize the processional order. The night of the rehearsal the groom's family will traditionally host the rehearsal dinner. To make sure that the groom knows what to expect and what his responsibilities are at the rehearsal and rehearsal dinner,
The Rehearsal
Everyone's chitchatting in their places, the priest is in his street clothes, your nephew is running willy-nilly through the church ... what kind of rehearsal is this? This isn't the wedding you've planned and imagined. Will things be so casual and disorganized tomorrow?! No. Think of this as a relaxed run-through. When tomorrow comes, you'll be able to feel the excitement in the air.
At the Church
Most churches have a scheduled time for rehearsals. What happens during your block of time? A mock wedding, basically. If you have a wedding consultant or if the church has a wed-ding coordinator (many larger churches do, just to keep things moving along as they should -- and your paid coordinator may have to defer to any rules the church has), she will instruct the bridesmaids on where to stand, how slowly they should proceed down the aisle, whether or not they'll be escorted by a groomsman -- that sort of thing.

