Back-up player = Players that enter the field very rarely. These could be youngsters, not needed players or in simple words - players that will play only if many players are out or rested.
Rotation player = Player that often enters the matches as a sub, maybe sometimes is in the starting eleven, but predominantly - a substitute
Regular player = A player that is a regular starter for the team and have participated in around 80-90% ot matches
Key player = Very important player, a player that could decide matches; Is the team captain or scores regularly.