Bairisch is the word for the Bavarian dialect which is spoken in Bavaria and also parts of Austria and South Tyrol. It was classed as endangered by UNESCO in 2009.
It is a dialect that completely baffles those from elsewhere in German and is even more difficult for non-native German speakers to understand.
In fact, Bavarian differs depending which area you are in. Nordbairish is spoken in Oberpfalz, Oberfranken, Mittelfranken and Oberbayern, Mittelbairisch in Niederbayern, Munich, Donau and Austria, and Südbairisch which is spoken in South Tyrol.
Although Bavarian can be written, it is primarily spoken; many media outlets throughout Bavaria use the Bavarian dialect.
Here is a video so you can get a taste of the differences between Hochdeutsch and Bavarian: