Style sheets and display methods

Can someone please shortly summarize the need for different types of styles sheets based on the display method in regards to CIS web page development? Plz help with any sort of information, it would be greatly appreciated.
Is there anyone that can help me out? PLZ!!!!
*sigh* I'm not asking for a very long summary or explanation or anything just need some info that will help me better understand the term, someone please help me out here I'm begging!!!!

One thing to keep in mind:
Depending on the site/target user it can be good to specify CSS sheets for screen, devices, print... but you'll want to be sure to limit the number of HTTP referrals to keep your page load speed fast... so maybe one style sheet (that imports the device and print css) specified in the HTML. There are pluses and minuses to every method and it depends on the situation.