The th table header tag is used to denote the first row(s) of a table: It (they) will be made up differently from the other rows in the table. Exactly how it is made up depends on the format. In printed documentation (latex) a line will be drawn under the row. In HTML, the font and background may be formatted differently.
The th tag can only occur inside a table tag, and can contain only td tags.
Example:
Will be formatted approximately as
| Cell (1,1) | Cell (2,1) |
| Cell (1,2) | Cell (2,2) |