The three classes of United States Senators are made up of 33 or 34 Senate seats each. The purpose of the classes is to determine which Senate seats will be up for election in a given year. The three groups are staggered so that one of them is up for election every two years, rather than having all 100 seats up for election at once. For example, the 33 Senate seats of Class 1 will be up for election in 2018, the elections for the 33 seats of Class 2 will take place in 2020, and the elections for 34 seats of Class 3 will be held in 2022.
The three classes were established by article e 1, Section 3, Clause 2 of the U.S. Constitution. The actual division was originally performed by the Senate in May 1789 by a lot, with a rule being that a state's two seats had to be in different classes. Whenever a new state subsequently joined the union, its two senate seats were permanently assigned to two different classes by coin toss, while keeping the three classes as close to the same size as possible
