Most people are familiar with Insert>Comment. Just right-click on a cell and select Insert>Comment. Very easy. Right-click on the comment to display it, edit it or delete it. You know there is a comment in a cell because of the red triangle in the upper right corner of the cell.

Using comments can be a pain as it is additional step and if you display the comment sometimes it can obscure other cells. A more interesting way to document formulas, in particular, is to use the N() function.
The N(item) function returns a zero when the item is a string so if I typed =Count(A1.A5)+N(“I am only counting numbers”) then the answer would be 2.=Count(A1.A5)=2+0 =2
This can be a nice quick way to provide information if you are creating a complex formula and want to remember what you are doing.

In the example below, I identified that I was looking up Unit Price based upon the Product ID   Now, if I had looked up Product Name instead of Unit Price, I would have received an error message if I had included the N() function. Can you guess why?  In that case instead of the resulting formula being 17.95+0=17.95 the formula would have been = “Bing Cherry”+0 =#VALUE!   You can’t add text and numbers together.  So, this may be useful, to some of you, if you use complex formulas and are always scratching your head wondering what the heck it says.