For 2×2, the notation is super easy. (These also apply to the 3×3.) There is a different letter for each face. Here’s a table of the notation:

Clockwise:

Front: F

Right: R

Top: U

Back: B

Bottom: D, because if it was B, then there would be 2 B faces

Left: L

Counterclockwise:

Front: Fi

Right: Ri

Top: Ui

Back: Bi

Bottom: Di

Left: Li

180 degrees:

Front: F2

Right: R2

Top: U2

Back: B2

Bottom: D2

Left: L2

    For 3×3, there are also middle turns, called M, S, and E. M is between L and R, S is between F and B, and E is between U and D.