Actually, it depends entirely on the lottery... You have scratch off tickets that are cheap, gives ya a few bucks if your lucky, you can usually cash them at the place purchased from. Then there are state, inter-state, and national lotteries. National level ones require you to be a citizen, state lotteries, you pay for the ticket, you win, you get cash, doesn't matter what country your in, as long as you purchase it while in america, and cash it in while in america. Paying in more will not give you more, you just get more tickets which increases your odds. If the lottery is funded by taxes and not ticket purchases, then you also can not claim a prize, as you have to be a US/State citizen to participate and get a chance to cash in.
The millions advertised are if you are the single winner with every number correct. If you get all but one right, you can also typically claim a prize of far less value. If there are multiple winners, the prize is split between them.
In the end, it comes down to which lottery your talking about, looking at, and wanna try to play in, every single one has different rules, but most all are the same. I have a ticket for the Kentucky Mega Millions ($363 million) that is being drawed tonight *crosses fingers tightly*. I swear if I ever won such a lottery, part of it would be split among the ACTIVE members here at that time.
The winnings also depend on how you take them, you always have to pay in taxes on it to the state/county you won it in, including federal taxes. Lets say I won that $363 million, if I wanted a lump sum, I only get a fraction of it, and the taxes removed. If I wanted it over time, I would get the "whole" $363 million, but pay taxes based on how much I get per year.
This is my states lottery
http://www.kylottery.com/apps/about_us/faq_home.html