What is the difference between congress and parliament?

Answer 1

A Congress, such as in the US, is responsible for making laws and works seperately from the part of government which executes them (which is led by the President). At elections, candidates for each party hoping to win control over a district are chosen in primaries before the subsequent election. Because of the emphasis on individuals, party discipline tends to be relatively relaxed. The members can't remove the President from power except with impeachment, which is only possible if he abuses his power.

In a Parliament, such as in the UK, some of the members work in the part of the government which executes them. This includes the Prime Minister, the leader of the ruling party, and the Cabinet, who all belong in the Parliament. As the Prime Minister must retain support of the majority of members (or else be removed by a "Vote of No Confidence"), party discipline tends to be strict. Political parties decide who their candidates are for each district, and their favourite nominees are often nominated for the safest seats.

Answer 2

It's the system of government. Countries which follow the American model of government have a Congress which makes laws and works seperately of the Executive Branch. Countries which follow the British model have a Parliament which makes laws but also controls the Executive Branch.