I have just a wee little question that I hope you can help me work out. If not, that's fine, I realize you're a busy man. In your series of A Song of Ice and Fire, what is the difference between a sellsword and a freerider?
Sellswords are mercenaries. They may or may not be mounted, but whether ahorse or afoot they fight for wages. Most tend to be experienced professional soldiers. You don't have a lot of green young sellswords -- some, sure, but not many. It's a profession a man tends to chose after he's tasted a few battles and learned that he's good at fighting.
You get more sellswords on the eastern side of the narrow sea than you do in Westeros. The Free Cities have made heavy use of mercenaries for centuries, to fight their endless wars in the Stepstones and the Disputed Lands. Over there many of the mercenary soldiers are organized into long-established sellsword companies, or free companies -- the Brave Companions are an example of such, though an especially unsavory one. You'll meet two more sellsword companies in A STORM OF SWORDS, the Stormcrows and the Second Sons. And there are others. The Golden Company is the largest and most famous, founded by one of Aegon the Unworthy's bastards. You won't meet them until A DANCE WITH DRAGONS.
Freeriders... well, that term is both broader and narrower. Narrower in that it excludes foot soldiers. You need a horse to be a freerider. Otherwise broader.
Freeriders are mounted fighters who are not part of a lord's retinue or feudal levy. Some are veterans, sure, but also green and untrained recruits, farm boys on ploughhorses, men dispossessed by the fighting, a very mixed bag. They don't as a rule collect wages. Some fight for plunder, of course. Other to perhaps to impress a lord or a knight , in hopes of being taken permanently into his service. For many it is simply a means to survive. If the war sweeps over your village, your house is burned, and your crops stolen or destroyed, you can hide in the ruins and starve, flee to the nearest city for refuge, take to the woods as an outlaw (the ones who do that are oft called "broken men")... or you can saddle your horse, if you're lucky enough to have one, and join one army or the other. If you do, you're a freerider. Being part of an army at least gives you a better chance of being fed.
There are all sorts of freeriders, ranging from wandering adventurers who are virtually hedge knights (lacking only the knighthood) to the aforementioned farm boys on drays. Most are used as scouts, outriders, foragers, and light cavalry.
Obviously, there is some overlap between the two terms. A mounted man who fights for pay could be called either a freerider or a sellsword.
Both terms carry a certain stigma in Westeros. Sellswords are said to have no loyalty, and freeriders no discipline.