SkyWest Airlines is a regional airline based in the United States. Unlike major carriers such as Delta or United, SkyWest does not sell tickets under its own name. Instead, it operates flights for major carriers such as Delta (as Delta Connection), United (as United Express), American Airlines (as American Eagle), and Alaska Airlines. So you are essentially flying with SkyWest, but under the brand of one of those major carriers.
SkyWest was founded in 1972 and has grown into one of the largest regional airlines in the world. They serve hundreds of smaller and mid-sized airports in the US, primarily connecting smaller cities to larger hubs.
The fleet consists primarily of Embraer and Bombardier jets (such as the CRJ200, CRJ700, CRJ900, and Embraer E175). These aircraft are smaller than the usual aircraft of major carriers, but offer a similar experience — just with fewer seats.
SkyWest plays a vital role in the US domestic network. Thanks to them, you can travel from a small town in Utah to the entire world with just a few transfers.
Although they operate mostly in the background, they are extremely important to the airline industry — both in terms of scale and reliability.