This is a repost from an answer I wrote a couple of years ago on Quora.
I usually fail miserably at explaining this for some reason.
Probably the most common definition out there is that management consultants are like doctors for companies (instead of doctors for people).
This analogy probably works for most cases. However, in the spirit of consulting analogies used by guru David Maister, there are probably sub-analogies. For example, there are some management consultants that are more specialized like brain surgeons. Brain surgeons work directly with patients, but possibly for shorter periods of time and using more specialized skills (and hence, brain surgeons charge higher rates). There may also be management consultants that operate more like pharmacists. In that regard, they may be more in the backoffice doing specialized work, interacting with patients less frequently.
Where the doctor analogy doesn’t always quite sit with me is that I usually think of going to the doctor either when something is wrong or for maintenance purposes. While it is true that a large bulk of consulting is about fixing company problems (e.g., unprofitable company, poor customer service, divisions not working together well, technology systems a mess), there are also management consultants that are more like life coaches or entrepreneurial partners that help with opportunities. These management consultants are not fixing things like doctors do. Instead they are helping companies build new capabilities (e.g., new line of business, new product) like a personal trainer helps a person develop more strength or cardio capacity.