The majority is dead skin flakes, but also fibers from tissue paper and carpet, and a tiny bit is stuff from outside, though the amount is very small - unless you live in a dry area with a lot of blown dust.
I heard somewhere that the average adult loses about 40 pounds of skin a year, though, of course, most of that is washed off in the shower. Ever scrape the soles of your feet while taking a shower? It's nasty.