Pasties are made from light,
flaky pastry, almost like
puff pastrty but slightly
denser. They are traditionally filled with a "base" of onions, potatoes, swedes and
gravy, topped with generous quantities of diced
steak. These days however it is possible to get them with various fillings, from chicken and
leek to the highly
unorthodox feta cheese,
pecans and spinach. Pretty much all pasties you'll see are shaped like a half circle with a thick braided crust along the rounded edge, and range in size from about 3 to 10 inches or so for the larger ones.
The best pasties I've had in the UK come from two shops: one in the Covered Market in Oxford and one in a small alleyway off Milsom St. in Bath. I don't remember what they're called, but believe me, you can easily find them - just navigate by the smell. Both shops make a large variety of vegetarian and vegan pasties, a great thing as it can be difficult for veggies to get hot food-on-the-go. The Bath pasties are frankly somewhat better, especially the giant steak pasty which is the size of my head and one of which will set you up for an entire cold winter's day.
Oh and by the way, the whole tin miners holding the pasties by the thick crust is apparently an urban legend.