Who is the officiant and what is his/her role?

The officiant is the person who performs the wedding ceremony. It's the minister, priest, rabbi, judge, etc. The person who legally pronounces you man and wife. Other duties may include: counseling the couple prior to the wedding day, preparing the wedding ceremony wording, attending and presiding over the rehearsal, and then performing the ceremony on the wedding day. They are typically invited to the reception. They are also typically paid for their services.