What is tag.?

Answer 1

In the html code tags are these <tag></tag> (of course replacing the tag name with something like p, div, img, etc...)

Answer 2

In Html context, the tag is a text identifier use to describe the content of the page... so it's required to differency the 'real' textual content from the additional information:

Basically to say "here start bold text" you write '<b>' and html parser know that's the descriptive part and not the content one ( so you need to 'escape' special characters in your content, at least the '<' should be write '&lt;' as all html tags start with one -- other unsafe character are '>', '&' and the quotes )...
In same kind of way, to say "here stop bold text" you will write '</b>'. Most of the time, we call 'tag' as well the pair, as one of the opening or closing tag.

Answer 3

Tag can be anything that is an elmemt such as li,p,b,td,div etc.