HTML is the computer markup language used to create Web pages.
CSS is a way of formatting text and layout in HTML documents. Using CSS, content information can be kept separate from formatting information.
Here is a really brief overview of HTML and CSS
Helpful References:
HTML/XHTML Tag Quick Reference – in pdf (U. of Albany Libraries)
HTML Cheat Sheet (psacake.com)
Tutorials:
Beginner HTML Tutorial from HTML Dog
Intermediate HTML Tutorial from HTML Dog
Beginner CSS Tutorial from HTML Dog
Intermediate CSS Tutorial from HTML Dog