Shimano had a number of groups over the years that either were short lived or were lower end but Richard has it mostly right other than he left Ultegra/600 out of the initial list. Additionally, like the MTB parts, there is a number assigned to each group and a letter code for each part in the group. For instance RD-6500 is a 9-speed Ultegra rear derailleur. The first number changes to reflect a jump in the group protocol: 5000 is 105; 6000 is Ultegra/600; 7000 is Dura-Ace. As the groups progressed through the years the second number changed to reflect the product's development: 6400, 8-speed Ultegra; 6500 9 speed Ultegra; 6600 10 speed Ultegra. There were exceptions but this will work as a guide line.