To usher means to go before, to guide, to herald.  John the Baptist was the usher for Christ.  To minister as an usher is to treat guests like Christ.  It is also to keep the service at the altar holy by maintaining order within the assembly; in addition, there are related responsibilities.  The ministry of an usher is outlined as follows:

Courtesy: welcome, place reserve signs, pointing out empty seats, helping elderly or others with various needs or directions, Making sure the elderly or pregnant have seating

Order: policing, keeping assembly quite, watching over children, checking rooms during service, keeping proper traffic Flow in Church for communion and processions, overseeing traffic flow outside of church during big feasts

Collection: taking the collection, organizing the collection after Liturgy

Peripherals: doors, air conditioning, lights