To follow up on a comment by Buckeye, I think it's important to differentiate between ADVISERS (who are usually associated with highly-talented players who have a strong chance to be drafted) and RECRUITING SERVICES. In fact, it wouldn't surprise me that some of these "Advisers" are simply calling themselves that because they don't want to be seen as the recruiting service that they really are.
True Advisers (in my opinion) are guys that already act as agents for current MLB or minor league players who have signed a contract of some sort with a pro team. They can "advise" high school players on many aspects of baseball (possibly including recruiting), but their ultimate goal is to help a player get a shot at pro baseball.
That same Adviser then becomes classified as an Agent for a particular player if he is representing the player in draft and/or contract negotiations with MLB teams.
On an added note for those who might have a potential draft choice in their family, keep in mind that the NCAA made a change last January in their rules that now permits HS prospects to have Agents represent them in contract negotiations with MLB teams. There are conditions associated with how the athlete and family interact with an agent in this situation, AND if the athlete chooses not to sign, but instead heads off to college, the agreement that the athlete and family have with the agent or attorney must be terminated before the athlete begins attending college as a full-time student.