Hot aka shortcut keys are combinations of keys you can hit to preform actions more quickly than selecting the option from the menu with your mouse. These are only ones I have memorized and use frequently to make my photoshop opertation much more efficient.

Ctrl + C - copy selected area.

Ctrl Shift + C - copy all layers of the selected area.

Ctrl + V - paste whatever you copied.

Ctrl + X - cut selected area.

Del - clear the selected area.

Ctrl + Alt + Z - go a step back.

Ctrl + Shit + Z - go a step forward.

Ctrl + N - new project.

Ctrl + O - open file.

Ctrl + S - save.

Ctrl + Shift + S - save as.

Ctrl + Shift + Alt + S - save for web.

Ctrl + P - print.

Ctrl + E - merge selected layer down.

Ctrl + Shift+ E - merge selected layer down.

Ctrl + + - zoom in.

Ctrl + - - zoom out.

Ctrl + A - select all.

Ctrl + D - deselect.

Ctrl + I - invert selection.

Ctrl + Shift + U - grayscale/desaturate selected layer

If you find this information inaccurate or have questions post in the forums