Majority are computer programmers/tech industry jobs. Many come from places like India, Russia, Eastern Europe.
Were the wages american companies like Microsoft pay them is at least 2 to 3 times what they would make at home, but still less than what Microsoft would have to pay in wages and benefits to american programmers. I'm sure Microsoft, Oracle, Cisco, etc.. are always the biggest backers of getting the number of H1B visas increased. THey always are.
I used to live near microsoft and there are entire apt complexs filled with indian H1B programmers. In the summer, they bring over their parents to visit, and the areas start looking like you've been dropped into a bollywood movie.
You can smell the complex (curry) from miles away too.