Rugby rules were originally codified in the 1880's, although game goes back to at least medievel times before Football/Soccer Rugby as we know them developed form (for instance) Football, Big Game, Murderball etc.
Back before then, its rules changed depending on the differning nations, local customs, generations, school house traditions etc and what pupils prematch agreed too - some of the public schools still have traditional school rules of those old times for traditional inter house/school rivalry settling of disputes/ethos/teamwork or plain tradtion blah blah.
Its quicker/ more fluid with less advert breaks and no 'timeouts' as such. Less players than AFL/NFL - same team for offence defence, 80mins match time, no quarters, joking refered to as a game of three halfs - two on the pitch, and one in the pub afterwards (if your old enough to drink that is).
A lot less stopping and starting - less time wasting (generally that is) with no forward passes - side and reward passes only, and lots of contact with little to no armour. In many ways a simpler 'pregenitor' of AFL/NFL rules aka oldschool 'Murderball'. Kicking/punts are allowed to force other team to play from further back - either from the catch position or from inline where it crosses the side line.
A Try (touch down) requires the ball to cross the opponants goal line fully with the player still in contact with it, it must be in physical contact between carrying player and the ground of within the oppnants goal area/TDZ, it is worth 7 points. and can be scored anywhere along behind the opponants goal line within the white line.
A caught ball in the opponants by the possessing team in goal area does not count as a try, its a penalty, the defending side gets the ball. A try canot be kicked across the line, it must be carried.
A Conversion is only a goal kick/punt between the uprights over the cross bar to count for 3? points - akin to a post TD kick.
A Scrum is sort of like the closed in scrimage line, but where they teams 'lock' together to brute force push the other team over away from the ball, or attempt too, to claim it for themselves if they aren't in possestion.
The ball can only be picked up from the scrum once the teams has passed over it leaving it at the rear of their scrum and 'open'. If the scrum folds/collapses it will reset, unless the this happens too much. Then the most offending/collapsing scrum/team will not be given the ball.
As there are no 'downs', the weight, force, skill, passing, kicks and penalties cause the teams to move up and down the pitch in manner like downs. Once a tackled player is down and the ball is in contact with the ground, he must release it, generally for anyone to grab it if they can.
The possesing team then defends the ball, trying to push the other team away from it to get/let another of their players hands upon it to play it. The opposing team tries to push them away from it to or grab it to claim it for themselves too,
There is 2 forms of rules, Rugby (Football) Union (RFU) and Rugby League - differing mostly in minor intepretations, throw ins, scrumage penalty details; aiming keeping the game more fluid for specators/media or more historical older rules.
I don't know all the rules or either version, but there's no offside rule IIRC, tackles involve no neck locks, bollock grabbing and no flying/jumping tackles unlike Aussie Rules Football - where almost anything goes if you get/swat/block the ball - they regually more loose teeth and noses in ARF than Rugby, where 'Coliflower Ears' are more the norm - also if you thought a QB looked a bit sissy reachin between a mans legs for his ball, then a Scrum looks positively worse and the ball is only put in afterwards, from the side - ooh er misses.
TPBM can clear up what I missed or got wrong methinks....