Subtle, ambiguous, and often unintentional, micro-aggressions are subtle, easily deniable. They seem to appear in three forms:
Micro-assault: an explicit racial derogation; verbal/nonverbal; e.g. name-calling, avoidant behavior, purposeful discriminatory actions.
Micro-insult: communications that convey rudeness and insensitivity and demean a person’s racial heritage or identity; subtle snubs; unknown to the perpetrator; hidden insulting message to the recipient of color.
Micro-invalidation: communications that exclude, negate, or nullify the psychological thoughts, feelings, or experiential reality of a person belonging to a particular group
Check here for more on Micro-agression Theory.