When no correct doctype available, browsers fail into quirks mode by default thinking this is an old page written in the times where there were no doctypes available.
It is limited and hard to format (needless to say) avoid it like the plague.
Keyword: best practices
Use one per page (ideally), this is your main title.
Use it for tabular data (spreadsheets and reports)
Shows inline (with no breaks) with other content around.
Used when you want to apply styling to just one element within a div or an area.
By being in a document, it implies a break before and after it
Use it to gain control over entire sections of a page with CSS. It can enclose other elements inside of it.