A style sheet is a special kind of notebook that defines a set of cell styles used in another notebook. The style sheet contains style definition cells, where each style definition cell sets the group of options that are applied to any cells of that same type. By changing a cell's style instead of assigning specific options to a cell, you apply all the cell attributes desired in one step. You also give your notebooks a consistent design and make it easy to change that design globally. (You can always set local options in a cell to override the settings specified by the style.)
A cell style defines various settings for the options associated with a cell, such as font, color, justification, alignment, and so on. There are many different cell styles in a notebook, such as text, input, and output, for example. The cell style can be applied to a single cell or a group of cells. Cell styles are assigned names that indicate their use, such as "Section" or "Text".
The style sheet can also define some global options for the notebook, such as headers and footers or page size.