I am trying to buy a Shimano cassette but am confused that they have a Dura-Ace, Ultegra, 105 line and at same time they have the HG-50, HG-80, etc line. Sometimes the gearing spread of an Ultegra can be the same as one in the HG-50 line.

I can understand the DuraAce, Ultegra, 105 heirarchy but how do they relate or differ from the HG line?