Well, when most people talk about Aikido they refer to the art that was started by Morihei Ueshiba. That's fine because the majority of Aikido can in fact be traced back to him. However, my first introduction to Aikido was through an organization called Nihon Goshin Aikido (NGA) and it claims no affiliation with Ueshiba or his family. NGA is characterized by using English names for the techniques, using Western styled attacks, being a bit rough compared to other styles, and having an intentional focus on self-defense. I currently study Yoshinkan Aikido. This style was started by Gozo Shioda who was a direct student of Ueshiba. It is also considered a hard style since we integrate strikes with our techniques. There is also a greater emphasis placed on having correct stances than one finds in many of the other styles of Aikido. It uses slightly different names for techniques than the other styles do (nikajo vs. nikyo for example). There is also a very strong emphasis on being able to use the techniques to defend yourself. The single largest group, headed by one of Ueshiba's descendants, is the Aikikai. I would refer to this a group rather than a style because depending on which dojo you walk into you can find anything from no-nonsense self-defense to people who are basically playing dress up and dancing around to any and all points in-between. There is also Shodokan which is also known as Tomiki Aikido. It was founded by Kenji Tomiki, another student of Ueshiba, and it has a strong Judo influence. It is the only Aikido style which has competitions. Another group is the Ki Society. They were founded by another student of Ueshiba's named Koichi Tohei. This group has the reputation of being mainly focused on the spiritual aspects of the art and having very little, if anything, to do with self-defense. I do not know if that reputation is deserved or not. I'm sure that I have forgotten some of the other styles. Hopefully someone can fill in the gaps.