Command Squad
A Command Squad is a squad of Space Marine veterans who accompany the Brother-Captain or other senior leaders such as a Librarian or Chaplain into battle. The exact name of the squad can vary from chapter to chapter, based on customs or tradition, but their function is largely the same. Serving in a command squad is a great honor as the members are often hand picked by the commander, not only for their combat skills but also for their tactical brilliance. Such a prestigious honor always speeds promotion within the chapter, thought this is not without risk. Accompanying the marine commander, the squad will often fight where the battle is the thickest and most wrought with peril.
The members of a Command Squad are known as Honoured Brothers, and members of a command squad are as follows: the Veteran Sergeant (which is sometimes not included), the Company Champion, Standard Bearer, and Apothecary, along with one or two veterans. The Veteran Sergeant will serve as second-in-command to the Captain, but it is rare for the Veteran-Sergeant to succeed the Captain as commander of the Company, though isn't unheard of. The Company Champion is the bodyguard of the Captain. While your average Captain is a human killing machine, the Company Champion will be charged with fighting enemy warlords and champions, allowing the Captain to better focus on coordinating the battle. If there isn't a Veteran Sergeant, the Company Champion will be the acting leader of the Command Squad when the Captain is absent. The Company Standard Bearer will be given the title of "Ancient" and protect the Company Standard from all harm. So if an enemy manages to kill the Standard Bearer and steal/defile the Standard, every Marine in the company will be honor bound to kill the one who did. So basically, the Space Marines were going to kill you, but now you had to go and make it personal. The Veteran will often serve as an adjutant or equerry to the commander.
The Company Champion will always enter combat with a combat shield and power weapon of some variety. The other members of the squad are allowed to take their pick of the company's armory. Unlike other veteran squads such as the Sternguard or Vanguard, members of the Command Squad (excluding the Champion and Apothecary) are not singularly restricted to ranged or melee weapons. However, they may not take heavy weapons.
Tabletop
For 100pts you get an escort unit of vets with reasonable options. The entire squad(with the exception of Apothecary and Champion) can take melee weapons and storm shields each in the same manner as Vanguard Vets. However they dont have access to special weapons. The squad's main strength is the FNP from the Apothecary. The squad's small size also makes it easy to fit into a Razorback or other transport with your leader.
A word of caution on the Company Champion. Your getting a WS5 marine with a free power weapon, however his combat shield only gives him a 6++ save and cant be replaced by a storm shield. Edit: Ork Codex set a precedent that states (Through official errata) that buying the storm shield before the purchase of the power weapon allows it to carry over. Cheers.