Cell[contents, "style"] represents a cell in a Mathematica notebook.
A Mathematica notebook consists of a list of cells.
You can see the form of a cell as an expression by using the Show Expression command in the standard Mathematica front end.
You can access cells in a notebook directly using the front end. You can also access the cells from the kernel using NotebookRead and NotebookWrite, or using Options and SetOptions on NotebookSelection[obj].
The contents of cells can be the following:
In any given notebook, a collection of possible cell styles are defined, typically with names such as "Title", "Section", "Input" and "Output".
Cells can have many options, including:
See The Mathematica Book: Section 2.10.1 and Section 2.10.7.
See also: Notebook, CellPrint.