Yoda is a fictional male in the Star Wars series. The character was portrayed on screen by Frank Oz ( Voice ). His allegiance / loyalty is to The Old Republic. Yoda, the 900 year old erm, uhm, well what he is, is never disclosed. Yoda is not the only one of his kind but they are very rare, there is another who sat on the Jedi Council called Yaddle, a female of his kind. All we know about Yoda is that is as mentioned 900 years old and a Jedi Master teaching many apprentices for 800 years. Yoda's back story might be revealed a little more when Disney produces the rumoured Yoda movie in and around 2016. For the original films, Yoda is a bit character but in the prequels, he is seen a little more. In the original series, he was controlled by animatronics developed in part by Henson, the puppeteers who are behind Farscape and The Dark Crystal movie.
Yoda is seen most at the Jedi Council providing advice in the unique way people are familiar with, 'do or do not, there is no try'. The only times we see him off Coruscant is when he visits Chewbacca on the planet of Kashyyyk which is under attack from the Separatists droid army.
At the end of the Revenge of the Sith, he duels with Vice Chancellor Palpatine in the Grand Senate Hall. After drawing the fight with Palpatine , Yoda escapes through a chute and is picked up Bail Organa. Yoda leaves Coruscant with Bail Organa to rendezvous with Obi-Wan Kenobi on the asteroid Polis Massa. During the meeting on Polis Massa, those at the meeting agree to hide Anakin Skywalker's children from him and Yoda flies off to Dagobah. It is not quite clear why Yoda chose to hide on Dagobah, other than it is on the outskirts of the planet, far way from the central worlds that include Coruscant.
In the prequels we see another of Yoda's species, a female version called Yaddle. Yaddle doesn't appear to have in any way a major part in the prequels and her character is quietly dropped. We never know whatever happened to her or if she's still alive.
Yoda's first appearance is to train Luke Skywalker after Obi-Wan Kenobi's ghost instructs Luke to visit Dagobah for training. Luke doesn't finish the training and wishes to help his friends on Bespin much to Yoda's annoyance. Luke Skywalker returns to Dagobah after he has helped rescue Han Solo from the frozen prison in Jabba, the Hutts palace on Tatooine. Yoda passes away before the completion of any training.
Yoda made a brief appearance in Star Wars VIII - The Last Jedi as a ghost to meet with his apprentice Luke Skywalker on Ahch-To. Luke said that the Jedi order should stop and when he tried to burn the oldest Jedi manuscripts, Luke found he couldn't do it. Yoda was able to burn the tree down even though he was dead.
The way Yoda speaks and the things that he has said is as iconic as his looks and what he has done. His talking is sometimes referred to as Yodaism.