Cascading Style Sheets (CSS) is used to manage the look and feel of web pages. If you change the colours or font, then those formats will change on every page that points to the CSS.
HyperText Markup Language (HTML) is used to create web pages.
​Javascript (JS) is used to add functionality or widgets to web pages. For example: showing your twitter feed on a web page.
Structured Query Language (SQL) is used to access information from a database. For example: Membership details when you sign into a webpage.