Position Definitions and Duties

Captain:
Captain was a senior line officer rank of most space services and usually given to starship commanders. On starships, the term captain is also often used as a synonym to commanding officer. Proper naval parlance says that, even if a vessel is commanded by an officer whose rank is lower than captain, that officer is still able to be referred to as a captain while on the deck of their own vessel, no matter their literal commissioned rank. Captains of starships, often located hundreds of light-years away from and out of communications with higher authority, must be able to function autonomously and make independent command decisions affecting Federation policy and countless lives. Accordingly, few Starfleet officers ever gain the captaincy of a starship, the result of decades of sustained excellence as a Starfleet leader. A senior captain given a large degree of responsibility or administrative assignment is sometimes given the title of fleet captain.

Executive Officer:
Executive officer is the term often used to refer to a vessel's first officer. In Starfleet it is frequently abbreviated to XO.
The executive officer is nominally responsible for the smooth running and organization of the ship / starbase, will manage many of the day-to-day administrative tasks. First officers will take command of the ship in the Captain's absence, also first officers usually lead away teams while the Captain remained on the ship.

Chief Helmsman and Helmsmen:
The CONN officer's operate the helm and navigation stations.

Chief Tactical/Security Officer:
Chief tactical officer is a senior position for Starfleet officers onboard starships and starbases. Tactical officers are in charge of tactical operations, including weapons and shields, and ship's security. The Chief of Security duties include the protection of Starfleet and Federation assets, the prevention and investigation of crimes, and personal protection of Starfleet officers and Federation or foreign dignitaries.

Assistant Chief Tactical/Security Officer and Tactical Officers:
The Tactical Officer is an officer stationed at the console that controls a ship's weaponry and shields. On Starfleet vessels, the tactical officer's job is to maintain weapon and shield readiness along with executing firing solutions. These solutions can range from a low intensity phaser beam or to plot a torpedo firing pattern. Security officer's duties include the protection of Starfleet and Federation assets, the prevention and investigation of crimes, and personal protection of Starfleet officers and Federation or foreign dignitaries. It is customary for landing parties to include a number of security officers as guards.

Chief of Operations:
The operations manager handles functions such as internal systems control, communications and sensor system usages. Furthermore, ops personnel coordinate the scheduling of resources, hardware and system usage for an entire starship, outpost or space station.

Assistant Chief of Operations and Operations Officers:
The operations officers handle functions such as internal systems control, communications and sensor system usages. Furthermore, ops personnel coordinate the scheduling of resources, hardware and system usage for an entire starship, outpost or space station.

Chief of Engineering:
On starships or starbases, the chief engineer is in charge of all engineers and engineering duties. The chief engineer's duties range from making the duty shift roster for engineering personnel to advising the captain on possible solutions for certain technical problems. In general the chief engineer is an officer with enough experience in his or her own field of expertise and with seniority over the rest of the engineers. A chief engineer is also a member of the senior staff and accountable for the overall performance of the vessel.

Assistant Chief of Engineering and Engineers:
Engineers are accountable for the overall performance of the vessel.

Chief Medical Officer:
The chief medical officer is a senior staff-level position, typically held by a lieutenant commander or commander who is a Doctor of Medicine with surgical skills. CMOs aboard a starship or starbase are directly in charge of the sickbay or infirmary, commanding medical department personnel and activities. The CMO's primary duties are the oversight and maintenance of the overall health and physical fitness of crew and passengers. In addition to the normal duties of an officer, the CMO also has the power to relieve an officer or crewman, including a superior one, of their duties if in the officer's professional judgment, he or she is medically unfit. Typically, the CMO is expected to have the support of a senior command officer and/or detailed medical documentation to justify this course of action. Officers likely to be assigned as the CMO over long-duration missions of exploration, have scientific training in disciplines outside of medicine, allowing them to take on major, extended research projects; or to better participate in a wider variety of encounters.

Assistant Chief Medical Officer and Medical Officers:
Medical Officers are charged with the overall health and physical fitness of crew and passengers. Officers likely to be assigned over long-duration missions of exploration, have scientific training in disciplines outside of medicine, allowing them to take on major, extended research projects; or to better participate in a wider variety of encounters.

Ships Counselor:
A counselor is a position aboard Starfleet vessels and installations, usually held by a senior Starfleet officer with training in psychology. Counselors are responsible for the mental well-being of the crew and civilian staff. The position is considered vital enough that it warranted inclusion in the senior staff of the Federation flagship; in that instance at least, the ship's counselor also had a diplomatic role, advising the Captain in first contact and other situations. As with the chief medical officer, the ship's counselor has the power to relieve other officers and crew members of duty if he or she feels that their patient is suffering from a condition that may hinder their ability to perform their duties effectively. At the Captain's discretion, the ship's counselor may be allowed to wear something other than their normal Starfleet uniform while on duty.

Chief Science Officer:
Science officers serve in the Science Department on starships or starbases in Starfleet. The head of the department is sometimes known as the Chief Science Officer and may operate the science station on the bridge or operations center. As the name implies, science officers are responsible for observations, research, and experiments in any of the sciences, including mathematics, statistics, physics (physical and temporal), chemistry, botany, zoology, geology, cosmology, and others. Some science officers specialize in a particular discipline, but the Chief Science Officer is expected to be familiar with most of the sciences and rely on his or her staff for particular expertise. The department has some overlap with the medical department in terms of microbiology, xenobiology, and life sciences.

Assistant Chief of Science and Science Officers:
Science officers serve in the Science Department on starships or starbases in Starfleet. As the name implies, science officers are responsible for observations, research, and experiments in any of the sciences, including mathematics, statistics, physics (physical and temporal), chemistry, botany, zoology, geology, cosmology, and others. Some science officers specialize in a particular discipline. The department has some overlap with the medical department in terms of microbiology, xenobiology, and life sciences.

Intelligence Officer:
Intelligence Officer: is a title given to an officer aboard a starship or space station who is responsible for intelligence matters. The officer typically keeps in contact with Starfleet Intelligence for the latest information. The personnel rank is that for the Starfleet Intelligence agency and only is consider an advisor aboard starships and space stations. The officer has no authority over field operations of any vessel or station and only provides assistance to the senior officers. The intelligence officer other duties is to collect any information that Starfleet Intelligence deems necessary or valuable. One famous individual that held the rank of intelligence officer was Worf for a brief period aboard Deep Space 9 behttp://www.startrek-horizons.com/forums/showthread.php?t=116fore later on becoming an Ambassador.

Source:
http://www.startrek-horizons.com/forums/showthread.php?t=116