WHO and WHOM are both relative pronouns; WHO and WHOEVER are used for subjects, and WHOM/WHOMEVER are used for objects. Consider:

The man who helped me was kind. vs.
The man whom I helped was kind.

In the first example, "who" is the subject of the verb "helped" - he was the one doing the helping. In the second example, though, "whom" is the subject of "helped" - he's the one being helped.

"Whom" or "whomever" must also be used as the object of prepositions: to who, for whom, with whomever, etc.

Pro-tip: if you're unsure of which one to use, try substituting "he" for "who" and "him" for "whom." He helped me makes sense, but Him helped me does not.

That said, the WHO/WHOM issue is not one that the GMAT tests very often. You're unlikely to see it nearly as often as other pronoun issues, such as singular/plural agreement (they/them/their vs. it/its) or demonstrative pronouns (that or those).