Yamaha keyboards are some of the most popular home keyboards, with the first Portasound keyboards appearing in 1960. Throughout the 1980s up to modern times, they have been in fierce competition with Casio.
The PSR series consists mainly of keyboards with a set of 61 full-size keys, while the PSS series tends to have mini-keys and/or less than 61 keys. The EZ series has light-up keys meant for easy learning, and the YPT series was originally an expert series, and while it's not clear what the letters stand for, it's probably something similar to "Yamaha Portable Teclado", with teclado being the word for keyboard in Spanish.
See also the Yamaha synthesizers.