The Control key, often abbreviated on your keyboard as CTRL, and located in the bottom left and right, is a pretty handy key.
For example...
CTRL and Z = Undo
CTRL and X = Cut
CTRL and C = Copy
CTRL and V = Paste
CTRL and B = Bold
CTRL and N = New
Also...
CTRL and A = All
CTRL and S = Save
CTRL and F = Find
CTRL and P = Print
Another use is with web browsers, hold the CTRL key and move your mouse wheel forward and back, the page will zoom in and out, awesome!