I use the following definitions when using these terms:
Guide: A person who possesses relevant activity specific competencies to lead or guide clients in that activity. There is no intention of imparting skills or knowledge to clients beyond that which is necessary to enable their safe participation in the activity. At the end of a session or program with a guide, the intent is not for the client to have acquired the skills to independently participate in the activity.
Instructor: A person who facilitates skill transfer or development to participants in order that they may act independently or with minimal supervision. This requires the instructor to be able to critique technique, apply a variety of appropriate instructional strategies and may require them to assess participant’s skill acquisition at the end of a program or session.
(These definitions are adapted from the occupational roles/functions within the Outdoor Recreation industry from the SR03 Training Package)
N.B. a guide may do some instruction on a program, but the outcome/intent is different.
Thanks to Dave Colin for the attached document from which I take my definition.