What political party does the leader of the opposition represent?

That depends. The party in power right now (2013) is the Democrats. Barack Obama is the leader of the Democrats. And as a result, the Republicans are the opposition party. Conversely, in 2000, George W. Bush got elected president. He was a Republican and thus, the leader of the Republican party. Because a Republican was president, it was now the Democrats who were the opposition party. So, the leader of the opposition is the man or woman who is considered to be in charge of the political party that is NOT in power.

Sometimes, that extends to the senate or the house of representatives: whichever political party won the majority of the seats during the most recent election, the Speaker of the House is from that party; and the opposition is the leader of the minority party. In 2013, the Republicans are in charge in the House, so John Boehner (pronounced BAY-ner) is the Speaker. His opposition comes from the leader of the Democrats in the House, currently Nancy Pelosi. But in the Senate, it's a different story. The majority in the senate are the Democrats, led by Harry Reid. And the Republicans, led by Mitch McConnell, are the opposition party.