A continuously variable transmission (CVT) has a nearly infinite range of gear ratios. In the past, CVTs could not compete with four-speed and five-speed transmissions in terms of cost, size and reliability, so you didn't see them in production automobiles. These days, improvements in design have made CVTs more common.
The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore turns at the same rpm as the engine, which improves both power output and fuel economy. CVTs became common in hybrid cars because they are considerably more efficient than both manual and traditional automatic transmissions, and their popularity skyrocketed from there as automakers competed for the best possible fuel economy ratings. As of late 2016, one out of every four cars sold in the United States was equipped with a CVT.
The CVT does have its downsides; most notably, it can be sluggish to drive, since it's engineered for efficiency rather than fun. However, as many drivers choose to move away from the manual transmission, which results in fewer manuals being offered, the CVT continues to increase its presence. The CVT also works best in small cars with small engines, which is why most trucks and large SUVs continue to use traditional automatics.
You can read How CVTs Work for even more information on how continuously variable transmissions work. Now let's look at a simple transmission.