Rolling elements are either balls or rollers. Rollers offer higher load carrying capability (theoretically, a line of contact vs. point of contact for a ball).
Retainers are used to hold rolling elements (balls or rollers) in proper orientation so they do not all bunch together. Typically made from stamped metal or molded plastic.
“Roller” implies that the part moves (rolls) when in use. Generally a roller is acting as a bearing, supporting a load along its outer diameter.