Free Sparring

Free sparring is trying to practice your techniques against someone trying practicing their techniques against you at the same time. This uncooperative practice is so that you can both practice realistically, as real attackers do not cooperate with you when it is time to defend yourself.  The free sparring round is spent practicing continuously without interruption. If your opponent is injured and can't continue in sparring, then your free sparring has failed because your practice hasn't continued for it's full length. 

Free sparring is then practice:

1. Continuous: not stop-and-go, uninterrupted practice with two non-compliant people and no judges or referees. 
2. Realistic: trying to make contact with the attempted techniques, uncooperative and not choreographed.
3. Safe: the point is to get better, any technique that results in long term injury - including any brain damage - is totally counter productive. If people in a free sparring round are swinging hard to slow down the other sparring partner in order to more easily survive the round, they aren't free sparring, they are fighting.

Here is an example of a common form of Tai Chi free sparring:

Notice that once the free sparring goes beyond what they are trying to practice they get back to practicing as quickly as possible. Also there is nothing choreographed about the practice, everything is highly improvised, they are really trying to get moves in on each other. In addition they are also not fighting: the coach is standing by but he's not saying much, and sometimes more proficient students are allowing other students to get advantages over them so the more proficient student can practice from a disadvantaged position. The participants are also being careful with each other's safety, allowing them to practice what they need to work on without being physically harmed. Same goes for this Kung Fu free sparring:

Light contact, continuous free sparring is good for practicing with strikes to the head. You can't really train your head to be more KO resistant aside from just having good head defense in the first place. However if you want to free spar with more intense contact to build toughness and practice landing higher impact techniques, one of the best ways is to free spar without attacks to the head. This practice is found in many styles of Kung Fu and Karate, and in boxing it is sometimes called "shoulder sparring":

In free sparring with weapons, the challenge is safety, because it is very difficult to control the force of weapon attacks. Most of the responsibility for safety in weapon free sparring goes to the choice of sparring gear:

Drills Are Not Free Sparring

A drill is a choreographed exercise between two people to build specific reflexes or other habits. Though drills can have different levels of resistance, they don't involve the improvisation required to build experience using the techniques against a real opponent. 

People who are frequently stopping to talk about every successful move in a round are not free sparring. Some fantasize they will be able to reliably stop a fight with one single technique, leading them to believe they only need to train to get in the first attack before their opponent harms them. This "see who can hit first" drill is not free sparring because it lacks continuous practice:

Another thing called sometimes called sparring that is not free sparring is any forms, katas, or drills that require two people to execute. For example this demonstration, as rich in skill and theory as it may be, is also not free sparring:

Sensitivity drills are an important exercise in many martial arts. However sensitivity drills are also not free sparring because they do not reflect any sort of realism. Even though there can be high resistance in these sensitivity drills, what is being practiced is not actual fighting technique:

Fighting Is Not Free Sparring

Free sparring is safe, you are not trying to knock out or injure your opponent. Unlike fighting, in free sparring you are safe to try moves you are not yet good at. This means there must necessarily be contact in free sparring, but that contact must also necessarily be a very safe level of contact, so that either participant can continue practicing. 

Dog Brothers and other gatherings are probably the single most important events going on in weapon martial arts today. These aren't tournaments, the gatherings don't officially have winners or losers, and no one is keeping score. However gathering participants are there to test themselves rather than to casually practice to develop skill, and there is not enough regard for participant safety to be considered free sparring:

This Tai Chi fighting tournament, as significant to Tai Chi as it was, is not an example of sparring, because people were fighting full force hoping to take the opponent out of commission:

Anything going on at a full contact martial arts tournament should not be considered free sparring, because they are fighting, doing everything they can to stop the opponent within the established rules (fighting to win, not free sparring for practice):

Free Sparring is Traditional

Based on everything I have seen and heard, traditionally most martial arts had free sparring. However by the mid 1980's many martial arts in the USA had become involved in a no-contact version of "kumite point-fighting" (the "see who can hit first" drill competed in Olympic-style Karate.) In the 1990's many martial arts revolted from kumite point-fighting by embracing kickboxing instead, and their sparring often had dangerously hard contact, over correcting for the kumite point fighting they had been involved in previously.

However by the 2000's many USA fighters had cross trained in Muay Thai kickboxing in Thailand.  USA gyms learned that even though Thailand has one of the most brutal reputations for professional fights, their fighters strongly preferred light contact free sparring while training. While free sparring now seems like a popular trend in martial arts, in reality free sparring is a return to ancient tradition. For example in traditional Karate one older name for free sparring is "Irikumi Ju":