tbody is the table body, the DTD allows multiple tbodys and says "Use multiple TBODY sections when rules are needed between groups of table rows". (By "rules" they mean solid horizontal lines, but I think you'd have to explicitly add any horizontal rules that you want via CSS)

Typically, you have one table header (thead) and one table body. In print, if the table spans multiple pages, you would want the header to re-appear on each page.