Historically, aikido is a budo that developed from old jujitsu styles particularly Daitoryu Aikijujitsu.
The techniques were passed on in detail from Sokaku Takeda to Morihei Ueshiba. With the changing times and his pursuit of knowledge, Ueshiba changed the name several times from Daitoryu Aikijujitsu to Aikibujitsu, Ueshibaryu Aikibujitsu, Aikibudo and others (the name changes and when the changes occured have not yet been clarified). After that the name Aikido has been used up until today so many people think that it is a new budo. However, looking at the organisation and basic structure, the atemi waza (striking techniques) and kansetsu waza (joint techniques) are inherited from old style of jujitsu.
For more detailed information see the following pages: History of Aikido, The Development of Aikido, and Organisation of Aikido.