They are basically used to format a display page from the data shipped by an XML document. In this manner a different XSLT could be used to format the data differently for deferent applications. The style sheets can be shipped along with the XML or stored locally.
With basic knowledge like that of HTML, CSS won't be an issue. I personally started with front end development and picked up php and SQL fairly quickly.
Best of luck. If I can help in any way, just shoot me a PM.