Sora - entry level 8 spd group - down fall is the poorly placed thumb shifter...and the fact that every other road group on the planet has at least 9 speeds.
Tiagra - the next step up - gets the 9th gear and moves to a shifter more like the rest of the Shimano line with a paddle behind the brake lever.
105 - the sweet spot in the Shimano line. For 2006 it has gone to the 10 spd and has the same mechanism design as Ultegra and DA but is a bit heavier. Crank gets the external BB design which you find now at all levels here and higher.
Ultegra - went 10 spd in 05 a little lighter and a little better finished than 105...really probably the highest level anyone that is not a pro racer needs.
Dura Ace - the flagship of the Shimano line. Went 10 spd in 2004 and some say is getting ready to go to as many as 12 speeds in the near future. A lighter lighter and a little better finished than Ultegra. There is also a rumor that in 07 DA will get some CF and Ti bits so that it cam compete with Campagnolo Record.

Personally I am a Campagnolo guy but I will say for Shimano that the 10 spd systems shift VERY nicely...the main thing I don't like about Shimano is the cable routing...sorry I am into aesthetics.