This article is from the Rugby Union FAQ, by roelofs@f2s.com (Mees Roelofs) with numerous contributions by others.
Arabian Gulf League: Teams from Dubai, Muscat, Doha, Kuwait, Abu Dhabi and
Bahrain contest the AGL each season. It runs from September through to
April. There is also a Gulf-wide knock out competition each season. These
nations also form an Arabian Gulf Team, which takes part in World Cup
qualications.
Celtic League: Five Welsh teams, the three Scottish district sides as well
as the four Irish provinces play in the Celtic League. The teams play home
and away between September and May.
European Cup: The European Cup (or Heineken Cup) is the Champions League of
European Rugby and runs from September to May. The top clubs and provinces
from the Six Nations are in it. The 24 teams are spread over six pools. The
six group winners and the two best-placed runners-up advance to the
quarter-final. The team ranked higher after the group stages will have home
advantage. The semi-finals and the final are played at neutral venues.
Parker Pen Challenge / Shield: 32 European teams not in the European Cup,
including some non-professional ones, compete for the Parker Pen Challenge
Cup. This is a knock-out competition with home-and-away matches being
played in every round, except for the final. The 16 teams that lost in
Round 1 will play on for the Parker Pen Shield, which has the same format.
The competition runs from September to May.
Super Twelve: The three professional state sides from Australia, as well as
five 'superprovinces' from New Zealand (made up out of multiple provincial
sides) and four from South Africa play in the Super 12. The teams play
eachother once during regular season. The top four advance to the semi
finals. The team ranked highest after regular season will have home
advantage for both semi final and final. The competition runs from late
February to early June.
 
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