Mercenary

A Mercenary is an independent soldier who has no official allegiance to any lord or country, but who instead hires out his services for payment. They are most commonly known by the term "Sellsword", as they sell their services with a sword.

The quality and reputation of sellswords ranges across a very wide spectrum: they are often professional soldiers, moving from one temporary contract to the next, though particularly poor-quality sellswords might just be a gang of untrained thugs. However, even sellswords who receive no "formal" training make a living by engaging in combat on a regular basis. Thus even a mercenary that is initially low-born and without expensive training will gain a great amount of combat experience ("hands on" training) throughout his career. Provided that he survives that long, a veteran sellsword may become a highly skilled fighter.

Songs of chivalry and romance often portray war as a glamorous series of duels between knights in shining armor, fighting for love, honor, and homeland. In gritty and dirty reality, a large extent of the day to day combat in real wars in both Westeros and Essos is fought by hired sellswords, fighting only for money.

Types of Mercenaries
While mercenaries are often referred to as "Sellswords" as a catch-all term, this is strictly speaking only one subdivision of mercenary:

Sellswords
Sellswords are professional mercenary soldiers. They usually fight on foot, though they can actually also own horses and fight as mounted warriors.

Notable Sellswords in the TV series:


 * Bronn - a weathered, veteran sellsword of no particular fame but above-average skill, who is hired on by Tyrion Lannister as his bodyguard/enforcer. Bronn first earned his place at Tyrion's side by defeating Ser Vardis Egen, a knight and the captain of the guards at The Eyrie, in a trial by combat, thus demonstrating his impressive skills as an experienced sellsword.
 * Daario Naharis - a sellsword from the Free City of Tyrosh, operating in Slaver's Bay.

Freerider
Although mounted sellswords are sometimes called "freeriders", strictly speaking they are not the same thing. The difference is that true freeriders do not fight for payment, receiving only a share of the food supplies to sustain them and the promise of a share of the plunder (they are "riders" that fight for "free").

Freeriders are usually a motley mix of two drastically different sources. Some freeriders are hedge knights, low-ranking freelance knights unattached to any noble House, and they are fighting for free because they hope that if they distinguish themselves in combat, they will be formally taken into a lord's service as a sworn sword. The other kind of freeriders are truly poor hangers-on, usually farm-boys whose homes were burnt out during the military campaign and who joined up with the army because they have nowhere else to go, and are only a step above camp followers. Either way, freeriders are rarely used in direct combat between armies, instead more often serving as scouts and foragers. They can see combat, but this usually takes the form of raids, burning down enemy villages as their own homes were once burned down.

Notable Freeriders in the TV series:


 * Lothor Brune - a freerider in the service of Lord Petyr "Littlefinger" Baelish of the Vale of Arryn.

Sellsails
Sellsails are mercenary sailors who engage in naval battles. Sellsails are the captain and crew of a boat, whose service is the actual operation of their vessel. Of course, the crews of many sellsword ships are also proficient in combat during boarding operations and coastal raids. Sellsails are quite frequently pirates, who have decided to make quick money by hiring out the services of their ships during a current conflict, and in return charging far more than they'd normally make in their usual raiding activities. During peacetime, they usually go right back to being regular pirates.

Notable Sellsails in the TV series:


 * Salladhor Saan - a flashy pirate-lord from Lys who commands a fleet of thirty ships, hired by Stannis Baratheon to supplement his own naval forces in the War of the Five Kings.

Mercenary companies
While many sellswords function independently of any larger organization, there are also many sellsword companies which hire out their services as a unit. Again, the quality of these sellsword companies varies across a wide spectrum. Some can be little more than poorly trained gangs of a few dozen men who have combined their strength. Major lords who want to supplement their armies with sellswords will prefer to hire a pre-assembled collection of sellswords rather than go through the trouble of hiring them one by one. The best sellsword companies are essentially small private armies, highly trained and serving under their own officers.

While sellsword companies are encountered in Westeros, they are much more common in the city-states of Essos, particularly in the Free Cities and Slaver's Bay. The Seven Kingdoms are predominantly a feudal society, in which each lord raises soldiers from his own lands. In contrast, the Free Cities are an urban society, and thus better fitted to paying coin to mercenaries. Thus, the Free Cities and Slaver's Bay have a much more developed system and history of fielding professional mercenaries organized into independent companies for hire.

Notable Mercenary companies in the TV series:


 * Second Sons