CSS stands for cascading style sheets. Its used as a way of defining display elements on an html web page (eg fonts, and their sizes... whether there is a border around pictures etc). If you do web design, you *need* to know CSS. Its easy to learn and implement.
If ever you want to know about any aspect of web design... go to w3.org... thats the organisation which determines how web pages will work and its then up to browser makers to make sure their browser can read the html.
Of course, you may have meant to ask about CSC which is something different and I will leave someone else to define it
CSC is shorthand for Compact System Camera. Compact meaning it is smaller than a DSLR and System meaning it has a range of interchangeable lenses. Examples of CSCs include micro 4/3 Olympus Pens and Panasonic G series, Samsung NX, Sony NEX and the new Pentax Q and Nikon 1 series. Welcome to the forum by the way.