Originally there were only two Rugby Positions—forwards and backs.
It was only when the rules were first drafted in the 1870's that the fullback, of which there were three, was named and his role defined.
A rule change limited the position to one player on the rugby field for each team. The decisions were then made that the other two players would be stationed at a midpoint between the forwards and the fullbacks and were to be called halfway backs.
In time this was shortened to halfbacks. Their role and that of the fullback continued to be in position to fall on the ball in the event of the opposition hacking it out of the scrum.
In 1878 at Cardiff, in Wales, they developed a short pass to one of the halfbacks who would then go charging ahead with the ball. He became known as the flying halfback which in time was shortened to the fly-half.
In addition they reorganized the scrum, developed shorter passes amongst the forwards and long passes amongst the backs. This lead to the need for more players to be placed in the back line between the halves and the fullback so they were called quarters and the fact that three of them were put in this position led to them being known as "three -quarters". The middle player being called the centre with the two on his outside called wings.
The introduction of a fourth player into the three-quarters was to a large extent, accidental, with Wales again being allowed to take the honor. Cardiff were due to play a tough match away from home and their first choice centre was not available so they promoted one Frank Hancock from the second side in his place.
Hancock was a great success scoring two vital tries. When the Cardiff selectors sat down to pick their team for the next match they were keen to revert to their original team, but they were most reluctant to drop Hancock, so they compromised by introducing a fourth three-quarter. Within two years Wales had introduced it at international level and the game became closer in positions to today.
The New Zealanders were quick to see the advantage of having a fourth player in the three-quarters. Their solution was to change the standard rugby positions by pulling a forward out of the pack and put him between the halfback and the three-quarters. Their problem was what they called the new position.
Legend has it that consent was reached by deciding that the halfback was 4/8ths and the three-quarters 6/8ths, so therefore the new position must be a 5/8ths, a name that has continued to this day in that country. When fly-half play developed they introduced the first 5/8th and the second 5/8th.
Here is kind of unobjective, humorous, but I think still useful information about the backs and the forwards:
There are eight forwards. They take part in scrums, lineouts, rucks, and mauls - essentially everything that is meaningful in the game.
The close physical work necessary for the Forwards engenders a sense of comradeship not shared by the prancing, self-centered and effeminate backs. The game has evolved, thank goodness, with multi-phase possession allowing the forwards to demonstrate their running ability with the ball.
Props and Hookers
The front row is the cauldron, the foundation for all good rugby play.The front row is noted for their power and good looks, like no necks, battered ears, and S-shaped noses. Not to mention their back-like throwing abilities.
Such players are noted for their intelligence and longevity well into their forties.
The second row is the engine room where the power flows. The second row, or lock forward, is tall, with plenty of leverage strength in the legs.
He must be productive in the lineouts as a jumper or supporter. Mobility is an added plus. The second row is much appreciated by the front row for their power in the scrummage.
The loose forwards
The loose forwards include the No. 8 and the flankers. The loose forwards are respected by the front row for their mobility, fitness, defense and support work (and their 'mungrel' factor).
The 'loosey' must have tremendous fitness and ball fetching instincts which are not completely understood or trusted by the props and hooker, who are perfectly content to scrummage all day for the ball.
The Scrum Half
The little scrum half provides the ball to the backs when the forwards are damn well ready for the strutting backs to knock it forward.
The forwards have grudging respect for the scrum half because he tries hard and is not afraid to get dirty with the rest of the scrummies.
The wise scrum half will drink and buy beers for the scrummies to maintain his favored position with the forwards.
It is rumored that the Fly has the best vision, hands, kicking ability, and overall tactical decision making ability on the side. (eg. Jonny Wilkinson)
The forwards do not understand or trust this individual. The hard-working scrummies generally expect and anticipate a knock forward from the Fly so they can have the pleasure of another scrum-down.
These hombres are supposed to be deadly tacklers, with strong running instincts and good hands.
Alas, the centers are lumped in there with the prancing fly-half. If the fly doesn't knock on, surely one of the centers will muff it up so the scrummies can experience the joy and satisfaction of another scrum-down.
The Fullback and Wingers
These guys may as well be from Mars. They are allegedly fast with excellent striking ability. They are supposed to score lots of trys, catch the high ball, and counter attack with flair.
But the stark reality (well understood by the forwards) is that the fullback and wingers prance and preen more than the inside backs.
They don't get dirty and are always playing with their hair and pulling on their collars.