You have the jist of it down, its any player with a body type that either limits their ability to play one of hte positions full time or puts them right in between the typical criteria for both positions. A combo guard is generally capable of both scoring and passing/running hte offense, and like a point forward, is often a star scorer who has the ball in his hands more than is generally usual for a player who is not a pure point guard because he is a. his teams best player, b. an above average distributor c. draws alot of defensive attention, getting other guys open.
Anyone who is two small to play the shooting guard position but scores like one, as well as anyone wiht a two guards body but has natural distributors instincts falls into this category.
Notable combo guards are guys like Dwayne Wade, who has the ball in his hands all the time for hte miami heat and averages 7 or 8 assists along with his 26+ ppg. He is generally considered to large/too scoring oriented to be a point guard, but out of necessity, and due to his ability to draw the defense and his passing instincts he has the ball in his hands most of hte time and all hte offense runs through him.
Another notable example is brandon roy, who is a scorer but can also distribute and often ran hte portland offense before they got andre miller and while miller is on the bench. Roy is a very responsible, low turnover player with no offensive flaws, who can handle hte ball and make plays for others but who is much larger than a prototypical point guard and has a shooting guards offensive repetoire.
When talking about allen iverson, the most usual formula for a combo guard, a guy in a shooting guards body who can distribute, is reversed, to a guy who can pass in a point guards body who can score like crazy.
Personally i would define Iverson as a combo guard, hes a guy who can score, pass, and have the ball in his hands every minute when he was in his prime, any guard who can average like 20+ points and 5+ assists who handles the ball alot and doesn't fit the general size criteria for either point guard or shooting guard could
be considered a combo guard. Basically, a point guard in a shooting guards body, a shooting guard in a point guards body, a hybrid point guard and shooting guard, or a guy in between the typical sizes for the two positions who has both point guard and shooting guard skillsetts.
Originally in the nba, there really were only guards, and then bob cousy came along and really revolutionized the point guard position. Even in the 80's and 90's there were backcourts with two combo guards or two poitn guards, or just two good all around guards that denied hte point guard and shooting guard molds.
combo guard is a very recent term generally used by scouts and talent evaluators.
hope this helped