Presides over the Senate, although the post is merely ceremonial, and is deferred to freshman Senators so that they learn Congressional procedure and protocol. This means they read roll calls and preside over Senate speeches, and begin and end sessions.
The Senate President Pro Tempore
The president pro tempore of the Senate is elected by the Senate, and by tradition the majority party elects their party member who has the longest seniority in the Senate. He is a voting member of the Senate unlike the VP. Most of the time who presides over the Senate is irrelevant.
When the VP is not present, the President Pro Tempore presides over the Senate. By custom, the Senate elects the most senior Senator of the majority party to be the President Pro Temp. What ends up happening though, is that the position it delegated to the majority party's junior Senators so that they can learn about how the Senate floor works.
The President Pro Tempore of the US Senate is the highest ranking Senator in the Senate, and the 2nd highest ranking official of the Senate (the Vice-President is the highest ranking official). Normally the Vice President is supposed to preside over the Senate, but in the Vice President's absence, the President Pro Tempore can preside over the Senate for the VP. However, in modern times, neither the VP or the President Pro Tempore presides - the duty is often delegated to Junior Senators to help them learn parliamentary procedure. Though the job is mandated by the Constitution, the President Pro Tempore is considered by many to be a largely ceremonial job. The President Pro Tempore is also 3rd in line in the succession to the presidency, after the Vice President and the Speaker of the House. The current President Pro Tempore is Senator Daniel Inouye of Hawaii.
The president of the Senate is the Vice President of the United States. When the VP is not there (which he usually is not), the Senate is presided over by the President Pro Tempore. This position is elected by the Senate and is usually the most senior member of the majority party. Currently, the President Pro Tempore is Daniel Inouye of Hawaii.
There is no Vice-president in that case. That office remains vacant and the duties of the Vice-president to preside over the Senate is then handled by the President pro tempore of the Senate.
The Vice President is the President of the Senate. While this job is often delegated to a Senator, to act as president pro tempore, the Vice President has the actual job. The responsibility of the President of the Senate, whether the VP or the president pro tempore, is to cast a deciding vote in cases of a tie. However, the VP can only cast a vote at that time, while the president pro tempore can vote in all occasions a normal senator can. Thus the president pro tempore essentially has 2 votes, with only one used in most cases.
The Vice President of the United States is the constitutional officer who presides over the Senate. The president pro tempore presides over the Senate if the VP is unavailable.
After the vice president it is the speaker of the house( currently Nancy Pelosi). After that President Pro tempore of the senate( the person who is president of the senate when the VP is not). After the president Pro temp it is each cabinet member in order of their creation.
The official titles of each presiding officer is: The Senate: Vice President The House of Representatives: Speaker of the House. When the VP is not available, they have a person who is called the President Pro Tempore.
The President pro tempore of the Senate becomes the President. He/she is the leader of the Senate when the vice president can not attend. If him/her dies also then the Secretary of State becomes President.
A president pro tempore is the leader of the Senate when the Vice-President is not there. (pro tempore is Latin phrase which means "for the time being".) Unlike the VP, he can also vote to create a tie, since he is a Senator. The position is elected by the Senators. Not so oddly, a member of the majority party always wins and it has become customary to choose the one with the longest seniority in Senate. The current (2014) President Pro Tempore is Patrick Leahy (D-VT).