Answer 1

All are different versions of same descriptive language.
Html1 a long time ago, next 2 ( the real first version ), 3, and Html4 ( 4.01 most commonly used since recent newest Html5 )...
Xhtml was another previous branch ( based on XML instead on SGML, itself base of XML, for main branch ), developed after/during Html4, which was abandonned for profit of Html5 after two great version specifications ( 1.1 and 2.0, even if second was almost dead before born ^^ ).