What is the difference I hear you ask?
Both a "Hit Point" model and an "Injury" model simulate the effects felt by a wounded character in a roleplaying game.
Hit points tend to measure the characters distance between two points of fully healthy and unconsciousness (or dead).
Injury models tend to measure how far a character has been pushed away from an optimal condition by tracking a series of specific effects on the character.
As far as most commentators are concerned, Hit Points are derived from the mechanisms of wargames. Early wargames began with players moving units of troops around a battlefield, gradually reducing the number of effective troops in a unit as conflict developed and damage was sustained. Once these games were reduced to a single player on each side, there needed to be a degree of granularity in the wounds inflicted...large games could easily reduce a soldier to an active or inactive status, but doing the same for a single character virtually meant that every strike was life or death. There was a desire to simulate small wounds, grievous wounds, mortal wounds. Thus a simple track was developed, in the form of hit points. The more hit points you had from your total pool, the healthier you were. The closer you were to zero, the more risk you faced.
There are dozens of hit-point systems. Some offer a fixed number of points for all characters (where tougher characters prevent damage before it reaches the hit point pool), some have a variable number of points (where tougher characters simply have a larger pool of points to play with). Some have only a single point pool to represent all types of damage (whether combative, poisonous, magical, exhaustive or otherwise), others have individual pools of points for each type of damage that can be sustained by a character.
Some "realistic" systems even have rules for blood loss, or system shock when too many hit points are lost in a specific period.
Injury systems are a more recent development in roleplaying, and they are probably more akin to the theatrical origins of live roleplaying than the skirmishes of miniature battlegames. Such systems apply a penalty to the character as they suffer more injuries, with more and more injuries applied until it simply isn't feasible for the character to perform actions any more (or until the character passes a certain threshold of injuries, then simply passes out or dies).
My personal preference at the moment tends toward injury systems, probably because they are newer, but also because I believe they have a better chance of reflecting a specific tone or mood of game.
Trying to track a range of injury types on a single hit point track can get very contrived in many circumstances. It works well for simplistic games, and it worked well within the original context, but the development of roleplaying has expanded beyond the original context. I'll have to track down some sources, but some of the original authors of Dungeons and Dragons have described their justification for using of a variable number of hit points (growing as characters improved in their experience levels). The justification states that originally hit points were a measure of how well a character avoided damage in conflict. More hit points didn't mean a healthier character, they just meant that the character was able to better minimise the impact of potentially deadly incoming blows. The hit point pool was never meant to be used as a resilience pool for poisons, psychic damage or magical resistance. These factors were added to it later.
It can be argued whether this was efficiency or laziness on the part of later designers. And in fact, this very topic has been argued a few times in my presence.
Some designs have specifically incorporated multiple tracks of hit points to reflect different types of wounds...physical damage and psychological damage tends to be a common one, but it could easily be argued that the health levels and blood points found in Vampire:the Masquerade provide a similar functionality.
As I get to this point of the post, I find myself starting to wonder about the systems which actually use injuries as character modifiers...many of them actually seem to have spun off from hit point systems to reflect incidents that a single track doesn't cover too well. Especially when combat brings in aspects such as called shots or hit locations, a broken arm might have the same chance of killing someone as a broken leg, but each of these wounds applies different types of penalties to the victim...A poked out eye does something different again.
There is no hard and fast rule indicating that a single system must be used in opposition of the other, and quite a few games try to take a bit from here, mixing it with a bit from there to get a system that hopefully meets the designers needs (there were lot's of these games in the 1980's and early 90's).
But as I indicated earlier, I'm tending more toward the pure injury systems now. Why does it matter how many hit points you have left, when the injuries are already telling you exactly how much harder it is to perform specific types of task.
I'd like to find a few more injury specific system to start