A fiduciary is a person or business that is duty-bound to put the interests of another, often called a "principal", before the fiduciary's interests. In addition, a fiduciary must also act as a "reasonably prudent person" when making decisions for, and acting on behlaf of, the principal.
Duty of Loyalty
A fiduciary must avoid conflicts of interest and self-dealing that result in a benefit to the fiduciary at the expense of the principal to whom a fiduciary duty is owed.
Duty of Care
A fiduciary must act to reasonably inform him/herself regarding a particular issue and must make a reasonable decision as to such issue on behalf of the principal.
Examples of fiduciaries include trustees, attorneys, title companies, trust companies, business advisors, stockbrokers, bankers, etc.
This brief overview of some important considerations associated with fiduciaries is by no means comprehensive. Always seek the advice of a competent professional when making important financial and legal decisions.