The Revolutionary War happened because the colonies were oppressed. They were oppressed by the British Government, which was ruled by a monarch who had absolute power. Once we were free, the last thing we wanted to do was to give power back to one single individual. So we separated governmental power over three branches and gave each branch power over the others. Enter one third of that triumvirate: The Legislative Branch.
The Legislative Branch has the power to create the laws which govern our nation. The power to create these laws has to be evenly distributed. For this reason, the Legislative Branch is separated into two branches of its own, the Senate and the House of Representatives. The Senate has 100 members. Two senators are elected from each states. Should populous states like California, have the same degree of representation as sparsely populated states like Wyoming? For this reason we have the House of Representatives. The house has 435 members. States are granted seats in the house based on population.
The Senate is a Little Special
Although things are supposed to be entirely balanced, the Senate does possess several powers which the House does not. The Senate has the ability to ratify treaties with a two-thirds supermajority vote. It is also the duty of the Senate to confirm the appointment of the President. The Senate is led by the majority leaders of each of the major political parties. Currently Mitch McConnell is the Republican Majority Leader, while Harry Reid is the Democratic Minority Leader.
Congress is the only branch of government that has the ability to declare war. Although the President is the Commander in Chief of the United States Armed Forces, any sanctioned act of war has to be declared by votes in the Senate and the House of Representatives.