CSS, or Cascading Style Sheets, are the way you can format your HTML to look pretty. CSS and HTML used to be considered separate technologies, but with HTML5 they are now combined as one skill set. Old HTML 4 formatting generally will still work in web browsers, but CSS should be used.
CSS3 is the current version of CSS and will be the version to be used for the foreseeable future. CSS3 and HTML5 have now been a standard long enough that even “old” bargain bin books can be used for reference.