The Vice President of the United States is the President of the United States Senate
In the executive branch of the US the Vice President holds the second-highest position right after the commander-in-chief. The Vice President takes office together with the president after an indirect election.
According to the US Constitution, the vice president is the President of the Senate, who controls the Senate's daily activities. In the absence of the vice president, the Senate's president pro tempore (and others designated by him) presides. As one of the Senate's constitutional officers, the Vice President votes only to break a tie.
By the way, if the president is not on a fit state to discharge his duties, he is to be replaced by the Vice President. If he is unable to take office as well, the speaker of the house takes presidential office. The Senate's president pro tempore is the third in line to replace the president. Fourteen vice presidents in the US history have later become presidents under different circumstances: the president's death, resignation or an immediate election after their vice presidential terms.