I like to know when something is a tag.
For example <p> is a tag, but < p> is not (usually displayed as text).

I would like to know if the definition of when it is a tag is defined by 
a version of (X)HTML, or if this is defined by SGML (for HTML) or XML 
(for XHTML).
Or is there no formal definition? And do browsers implement its own 
ideas of when something is a tag?

Could you point me in the right direction for this specification?

I hope this has not been off-topic.

Robert.