HTML/XHTML Standard Rules

Hello.

Hey everyone, would any of you happen to know where I can find the basic rules for XHTML? I have one in particular (that, if could be solved here too would be very helpful) concerning text.

Text must always, always be placed either within a paragraph tag, a span tag (even if the span is empty aka no attributes?), an li, or a heading (h1, h2). Do links have to be between paragraphs? Is what I just stated there not true, or must I wrap everything with - for example - the paragraph tag?

And where would I find this, say on the w3c webpage, and other in-text standards (i.e.
 and such for ... I forget what they're called, sorry, but elements or attributes that don't have a start and an end)?

Thank you!