The USA has no Prime Minister as it is a republic, their president is Barack Obama who's sort of president of the world as so much depends upon the USA for the whole world, but there is no Prime Minister
Parliamentary democracies elect members of Parliament, who select the Prime Minister; he/she will select ministers to perform the administration of laws passed by Parliament. Differences between president and Prime Minister: There is not direct election of the Prime Minister, and he is almost never from a minority party. The majority party will select the PM, who continues to serve until the majority party loses its majority status, or he loses a vote of confidence. In England, the PM holds question time every Wednesday, and this is typically televised. Ministers may answer questions once a month, and this time is how suggestions may be made. . In America, Presidents are directly elected and serve for 4 years at a time, two terms maximum. The president is sometimes not from the majority party (the party winning the House), and this results in divided government. This is called a Constitutional Democracy. Obama is the President.