A number of alterations can be made:
1. Side seams and back seams -- The side seams and back seam in the torso can be taken in or let out to make a wedding dress fit a bride. This is true even for gowns ordered to size. Wedding gowns are usually made to allow the side seams to be let out up to 3 inches or 1-2 sizes (Depending on the gown). Additionally, the back seam often can be let out slightly to size up further.
2. Shoulders -- It is also common for shoulder seams to be taken in to bring up a gown.
3. Hem -- The length of a dress can be shortened only and is affected by the height of the shoe you are planning to wear. Some gowns can only be shortened from the waist. Other gowns can only be shortened by taking up the hemline.
4. Bustle -- The bustle is a way to hook the train up in the back so that the dress can be worn comfortably after the ceremony. Bustling a dress is part of the alterations for any gown that has an attached train.