Dojo is a Japanese term that generally means a place where someone practices something for purposes of mastery. An aikido dojo is basically a gym where the martial art aikido is practiced. In a more abstract sense, an aikido dojo could be any location where people dedicate themselves to practicing aikido. There are several varying schools of aikido with different approaches to training, and there are many aikido dojo locations all around the world.
Aikido was developed in the early 20th century by a jujitsu practitioner named Morihei Ueshiba. He was traditionally trained in many classical forms of jujutsu, which is the samurai hand-to-hand combat art, and he also had a knowledge of several weapon-based fighting systems. Ueshiba was a very spiritual person, and this spiritual inclination was generally applied to the philosophy behind aikido.
The art is generally very circular in nature, and practitioners usually avoid meeting force with direct contradictory force. They will often turn on an oncoming opponent and attempt to harness his momentum to disrupt his balance. This can require relatively precise timing and perfect execution. For this reason, Aikido is known to be more difficult than many other martial arts, and it can take significantly longer to reach a high level of proficiency. The overall focus of the art is generally quite peaceful in nature, and most of the techniques are designed to disable an enemy without actually injuring him in any way.
Someone looking for an aikido dojo will find that there are many different approaches to teaching the art. Some of the styles are so different that they may almost seem like entirely different systems. This is generally because Ueshiba varied his teaching significantly during his lifetime. Students who studied with him at different times were learning very different approaches, and when those students went out and opened their own aikido dojo locations, they brought these variations with them. For example, some of the schools may emphasize simplicity and self-defense, while others may emphasize more spiritual or esoteric aspects.
Aikido is generally considered a relatively popular martial art, and there are aikido dojo locations all over the world. According to some experts, the quality of teaching can vary significantly, and people are often urged to be cautious about choosing an aikido dojo. Some of the things that might be considered when searching for a dojo would include the price of lessons and the teaching approach of the particular school.