Risk is a board game, variations of which can also be played online, as for instance on Dominating12, Conquer Club, or MajorCommand. Though the gameplay mechanism is consistent, there are many variations between these websites - occasionally extending even to the rule terminology. As a general reference therefor we would recommend the succinct Dominating12 terminology glossary.
While we try to accommodate for rule variations in the following instructions, please be aware that they are meant specifically for 2-player (1v1) games (optimal gameplay is very different for multiplayer games). We follow the most popular battle mechanism, where the attacker rolls up to 3 and the defender up to 2 dice (with the defender winning ties). Our notation for attack configurations is AvD, with A being the number of attackers (troops on the attacking territory minus one) and D being the number of defenders (total troops on the defending territory).