In the United States Military (and that of many other countries that use similar ranks derived from the British and German (Prussian) military structures), the highest ranked officer is a General. Generals come in several forms, from lowest to highest (O-Numbers are pay grades, plain numbers are number of stars forming rank device):
(For an explanation of other ranks, see United States Military Ranks.)
Any officer holding one of these ranks is considered to be a "general officer," meaning that his or her career no longer involves leadership in a particular field but in the military at large. In the Army and Marine Corps, when one is promoted to brigadier general (and sometimes colonel), an officer loses his or her military occupational specialty (MOS) (for example: infantry, military intelligence, aviation, logistics) and is redesignated as a general officer.
All generals have stars on their collar as rank insignia. Up through general (O-10), these stars are added in a straight line; Five star generals have the stars arranged in a pentagon, a fitting shape considering the building that houses the headquarters of the Department of Defense.
General officers are responsible for large-magnitude strategic leadership and organizational decisions that affect entire divisions, corps, or numbered air forces. By the time officers are promoted to one of the general officer ranks (after serving in the lower-ranked company grade officer and field grade officer echelons), they have attended their service's War College and have a thorough comprehension of the scope of their responsibility.
In the Navy (because the Navy always has their own way of naming things), officers in similar ranks (e.g. admirals) are referred to as flag officers -- but most of the above paragraph still applies.
* I hear you scream, "Lieutenant comes after Major? What you say!! That's not right!" And I say in reply, "Au contraire, mon frere. Get the skinny under Lieutenant."
** These ranks are only assigned in times of war; see their respective nodes for more details. It is interesting to note that while the United States Marine Corps follows the same officer rank structure as the Air Force and Army, they do not have a "General of the Marine Corps" rank; the Commandant of the Marine Corps will report to the Fleet Admiral in times when one is appointed.