6 Nations started as an annual British teams tournament (4 Nations), then France Joined (5 Nations) then Italy (6). It's now all the major teams in the Northen Hemisphere. The southern Hemisphere has the Tri Nations (Aus, SA, NZ) which is similar. Argentina is due to join that soon. Those two tounaments are both annual and cover the main rugby nations. There are smaller tournaments for growing rugby areas such as Japan.I just don't get how there are like 50,000,000 different tournaments. Six Nations, The Sevens, World Cup, etc.
World Cup follows the same format as football, every 4 years.
The sevens is another code of rugby though has parallels with Rugby Union, designed to be a fast paced spectator friendly game that can be played in short tournaments (will be in the olympics soon I think). Very fast paced game with 7 minute halves. Tends to be dominated by south sea island teams.