Most warranties don't cover anything that needs regular replacing, like wiper blades, oil, tires, or brake pads. That is pretty standard.
Other parts, like the transmission, water pump, or alternator, may wear out but are not expected to. So they are usually covered in a "bumper to bumper" warranty.
Surprisingly, most "bumper to bumper" warranties do not cover interior trim, seat leather, etc. For some reason, they are often left off the warranty as "non-mechanical" parts.
Lastly, make SURE that you are getting a reputable warranty if you decide to get something extended (usually anything past 3 yrs/36,000 miles). Just because you buy a warranty at the dealership does NOT mean it's a factory warranty - I got burned on this by a GM dealership once. You don't want a 3rd party warranty, unless you are very familiar with it and are sure that it is legitimate.