The United States of America is the oldest modern democracy in the world, spanning from 1776 to the present day. It has three branches, the Legislative branch (also called the United States Congress), which includes the House of Representatives and the Senate, the Executive branch, which includes the president, the cabinet and the departments and agencies, and the Judicial branch, which includes the justice system. Members of the Executive branch don’t have seats and cannot participate in the Legislative branch, nor be removed by them.

Similar to Australia, the US has two major parties, the Republican Party (Republicans) and the Democratic Party (Democrats). Unlike Australia, there is no opposition leader, as such, just presidential candidates from different parties. This means the Democrats don’t currently have a leader, though they have two current presidential candidates, Barrack Obama and Hillary Clinton. They are chosen by votes in different states. There are also minor parties in the US political system, including a Green Party.

In the US. Presidents can only serve two four year terms. This means that if a president’s party wins an election for a third time, the president will have to retire and a new candidate has to be chosen. For example, even if the Republican Party wins in the 2008 election, George W Bush has to retire and would be replaced by a new candidate, in this case, John McCain.

In one rare case, a US president has served for longer. In the 1940 election, Then president Franklin D. Roosevelt’s Democratic party won the election, but as it was during World War II, he was elected again as it was too hard to find another candidate. He was also elected in the 1944 election, but died a few months later.

http://wiki.answers.com/Q/How_are_the_presidential_candidates_chosen
http://www.britannica.com/eb/article-9109502/Franklin-D-Roosevelt
http://australianpolitics.com/usa/overview/
http://www.humanities.mq.edu.au/Ockham/y67xan1.html