What is it?
Cascading Style Sheets (CSS) are the bit of the internet that makes webpages beautiful.
In the old days you would add colours (or colors, to use the actual US-oriented version of the web, bizarre seeing as Tim Berners-Lee is from our motherland) to the actual HTML tags in web pages.
Over time content and design started to be separated, with the structure of web pages remaining in the HTML, whilst the design was moved into separate tags, and then separate files, called stylesheets with a .css file extension.
An example of CSS code would include a tag class (with a full stop (period) before the name) and then a definition of which attribute you want to modify, followed by the value to be defined:
.classname {
background-color: red;
}