At some point, a Jedi Knight becomes a Jedi Master. He takes on a Padawan Learner or by virtue of his accomplishments becomes recognized as an example of the Jedi Order.
The title brings with it a few more responsibilities, but overall, life as a Jedi Master is not terribly different from life as a Jedi Knight. The Jedi Master still undertakes tasks and carries them out as best he can, though he has greater latitude in his means of accomplishing them. He also speaks with more authority then a Jedi Knight. A Jedi Master is afforded a great deal of respect within the order, regardless of how long he holds that status.. The station of Master has no "ranks" though some Jedi Masters have more authority than others due to personal reputations.
Most Jedi Masters downplay their own importance, except when they feel the situation warrants a perception of higher authority then a Jedi Knight commands. Most Jedi Masters feel that excessively emphasizing the distinction is a sign of vanity. Even so, Jedi Masters generally make it clear when they speak on behalf of someone else, be it another Teacher, the Council, or someone outside the order they are assisting at that time.
Technically speaking, no position in the Order ranks higher then that of Jedi Master. Traditionally speaking the rank of Grand Master is reserved for those Jedi Masters that have distinguished themselves within the Jedi community and on the Council. Masters Yoda & Mace Windu where the only two known Jedi Masters to attain this rank. Though in time a Jedi Master might become a member of the Order's Council. Ordinarily, a Jedi Master is expected to speak and act as a though he did so on behalf of the entire Order, if a Jedi Master truly knows the will of the Force, he does so.