In the United States Senate, a Senior Senator is a Senator who has been serving longer than the other Senator from the same state. Because each state sends two Senators to the Senate, there are 50 Senior Senators at any given time, along with 50 Junior Senators. Although there is no technical difference between Senior and Junior Senators, by convention, Senior Senators have much more clout, and they tend to get more attention from the media.
If two Senators from the same state are elected at the same time, the Senator who was sworn in first is considered the Senior Senator. More commonly, one Senator retires while another remains in service, and the replacement for the retiring Senator becomes the Junior Senator. As a general rule, Senators from the same state work together, because they have similar interests and goals, and a Senior Senator may mentor the Junior counterpart, often grooming the Senator for the day when he or she will take over the Senior position.
Senior Senators do get a few special perks. They are allowed to pick Senate desks before their Junior counterparts, allowing them to sit closer to the head of the Senate Chamber, if they so desire. They are also given preference on committees, and they may be treated with more respect by other Senators. A Senior Senator tends to have more political power, especially if he or she has served several terms and comes from a large state.
Generally, Junior Senators don't have a major impact on policy, because they lack the power and the experience to get support for proposed legislation; instead, they use their Junior terms to build up experience and learn about how the Senate works. There are a few exceptions; Joesph McCarthy, for example, was a Junior Senator who ended up having a huge impact on the Senate, as was Lyndon B. Johnson, who later became President of the United States.
Although Senior Senators often have more experience than Junior Senators, by virtue of their longer terms in the Senate, this isn't always the case. A Junior Senator might have worked his or her way through a variety of political positions from the local to the national level which provided a great depth of experience in a variety of venues. Sometimes, a Junior Senator is also more in touch with the home constituency, since the Junior Senator may have served on a local level more recently than the Senior Senator.