The top level of competition in the Welsh rugby union system is the RaboDirect Pro12 (historically the Celtic League) which is a league of professional teams from Ireland, Scotland, Wales and Italy. Wales currently has four entries in Pro12: Cardiff Blues, Newport Gwent Dragons, Ospreys and Scarlets.
Pro12
This is a closed league with no promotion or relegation; teams are effectively franchises. The teams also compete in the Heineken Cup, the Amlin Challenge Cup and the LV= Cup.
References
Welsh rugby union system Wikipedia(Text) CC BY-SA