The Format paintbrush copies everything related to the formatting of a selected cell. When you select a cell and click Format, you can then highlight a whole range of cells, and each one will take on the formatting of the original cell, without changing their values.
The Clear button lets you either clear the value, or just clear cell formatting.
That said, other formulas that reference a cell will take into account the current value of a cell. If A4 = A3 + 1, then it would be equal to 4, because it stacks the formula of A3 (A1 + A2) with A4 = A3 + 1. Formulas can reference other formulas multiple times.
Conditional formatting is a bit more complex. Use the drop_down menu to select from a range of options, like inserting helpful visual icons to represent status or completion, or changing the color of different rows. Most important are the conditional rules, which are created with a simple logic. For example, lets say you have a column with data in A1 through A3, and A4 holds the sum of these three cells. You could place formatting on A4 with a rule that says “if A4 > 0, then highlight A4 green.” Then, you could add another rule that says “if A4 < 0, then highlight A4 red.” Now you have a quick visual reference where green = a positive number and red = a negative number, which will change based on what you enter into A1, A2, and A3.