When you click inside the input area of a cell you enter edit mode. This is indicated visually by a shadowed box surrounding your cell. While in edit mode you type text normally into the cell. Many keyboard shortcuts are disabled.
You can enter edit mode from command mode by pressing Return ⏎ from command mode, or by clicking inside the input area with the mouse pointer.
In command mode there is a simple dark outline of the cell. While in command mode you can quickly perform many operations with a single keystroke.
You can enter command mode from edit mode by pressing Esc, or by clicking on the cell outside the input area with the mouse pointer.
⇧ Shift + ⌘ + D: delete cell
⇧ Shift + ⌘ + O: move cell up
⇧ Shift + ⌘ + L: move cell down
⇧ Shift + Return ⏎: evaluate cell and move cursor down
⌘ + Return ⏎: evaluate cell with no cursor movement
Option + Return ⏎: evaluate cell and insert new cell below
⇧ Shift + ⌘ + X: cut cell
⇧ Shift + ⌘ + C: copy cell
⇧ Shift + ⌘ + V: paste cell
Y: change cell type to code
M: change cell type to markdown
R: change cell type to raw
K: select previous cell
J: select next cell
A: insert cell above
B: insert cell below
X: cut cell
C: copy cell
V: paste cell
⇧ Shift + V: paste cell above
1: heading level 1
2: heading level 2
3: heading level 3
4: heading level 4
5: heading level 5
S: save
D + D: delete cell
Z: undelete cell
L: toggle line numbers
O: toggle output
⇧ Shift + O: toggle output scrolling
I + I: interrupt kernel
0 + 0: restart kernel