It is hard to find a definition with which everybody agrees, and there are different types of counselling. However, the following are widely accepted features.
Counselling aims to help the victim to adjust to a traumatic event. (It is not a "treatment" for post-traumatic stress disorder [PTSD].)
The counsellor will generally help the victim to find his/her own solutions to problems which the trauma has produced (rather than telling the victim what to do).
Counselling also provides an opportunity for victims to express their feelings, fears about and reactions to traumatic experiences.
Unfortunately, counselling has attracted justified criticism, particularly after major disasters when some counsellors behave as though everybody will need such help after these events.
It is important to bear in mind the natural resilience of the individual and the invaluable help which can be provided by family, friends, colleagues and communities.
Unless there are grounds for assuming an individual is at particular risk of a bad reaction to a trauma, then "watchful waiting", following some brief "psychoeducation" (i.e., about normal reactions and self-help measures) and advice is probably the best policy to ensure that professional help, prematurely provided, does not get in the way of the normal methods of adjustment and healing.
Also, there are concerns about the training, experience and qualifications of those who describe themselves as "trauma counsellors".