What does double and triple gear shifters mean? I am looking at these components and am wondering what does double and triple gear shifting mean?

Shimano R221 Double Gear Shifters 8 Speed
http://www.chainreactioncycles.com/Models.aspx?ModelID=38724

Shimano R221 Triple Gear Shifters 8 Speed
http://www.chainreactioncycles.com/Models.aspx?ModelID=38725
Double and triple refer to the number of chainrings (front gears) that you have. 

Here are the detailed service instructions, which allude to this fact:

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/NonSeries/ST-R221-R225/SI_6LU0A_En_v1_m56577569830606213.pdf