Note tool in Photoshop can be found in the toolbar though the exact position may vary depending on customization of your version of the application
What does the note tool do ? It adds messages as well as moves them and allows you to select them and change them as well as delete them. On saving the document in PSD or PDF, the details of all these entries are saved and restored when you next open the document. It adds an author as well. Sadly, it doesn't store images or snapshots or voice messages nor does it store the date / time (which would have been useful) unless you add it manually
Select it and then you will see the options (along the top) includes tool presets as well as the author to be added to the message as well as the color (yellow by default but it can be changed to the same color of the image etc). The text you want to add is not added on the control bar but in the panel.
Click the image to add a message in a particular location. Once you have done that you will see a little icon appear and you can enter the details in the notes panel
It is given a number 1/1 or 2/2 or 3/ 3 depending on how many already exist. Sadly the entry on the screen does not show the number. As you scroll through them (say there are three) and you go from 1/3 to 2/3 to 3/3 etc using the arrow keys (left or right) on the panel then you will see a little change to the icon, a pen will appear on top of the post it. As you change to the next one, the pen will move to that.
You can also, if they are annoying, move them using the move tool over to the side of the document. If you hover over the post it, it just says who the author is and not the message itself, which does seem slightly baffling as it would be more useful to see the actual details.
You can right click the entry. There are a number of options available.. If you are adding a new note then new and import and delete are available. If you are clicking an existing then open etc is available.
There is a clear all message in the control bar which is the same as 'delete all' (why are they not consistent naming conventions) There is show / hide but that does not hide the actual messages, it just hides the panel (pity as it would be great to have a hide / show for those)
They are saved in the PSD format file as well as PDF (there is a check box for that).
So what can you put in them, well, anything, they are useful if you are working in teams as a reminder of what needs to be done, what has been done, who did what work etc and even for a single artist, useful as a reminder such as where models or designs were imported or perhaps effects added (I would love to see that as an automatic feature) especially when you come to an image years and years later and a little post it would be a great reminder how you created an effect etc or the library used.
There is a presets feature in the top left of the application but it doesn't do very much. You can't use the presets to save a preset with a particular message etc (say if you have a particular client that you regularly do work for or a particular thing you generally set a reminder for or a repeated task). You can create an action to add a message but the action does not record the actual details of the post it, which may or may not be useful if you have a set of automatic reminders that you would like to add.