Palettes are simply Notebooks embodying functionality that may be usefully applied to other Notebooks.
Palettes, though they are Notebooks themselves, are not editable while they are being used as such. However, unless specific steps are taken prevent such an occurrence, opening a Palette as a Notebook will allow the modification of its content.
As stated in the previous paragraph, any Notebook may be opened as a Palette. This is done by selecting the menu item "Palettes->Install Palette" and browsing for a Notebook. However, the distinction between Palettes and Notebooks remains in effect. To reiterate, any Notebook opened in this manner shall not be editable for the duration of its use as a Palette.
Palettes provide collections of functionality that is often times re-usable across Notebooks. There are even some Palettes available that implement a reasonable facsimile of tabbed user interfaces for Notebooks. A feature that, for some reason, does not currently exist within Mathematica as shipped.
Regardless of personal observations, any time that, no matter which Notebook you are using, you find yourself repeating the same steps that functionality becomes a candidate for encapsulation within a Palette.