Pin | JoJo's Bizarre Adventure (see variants)

JoJo's Bizarre Adventure, often shortened to JoJo or abbreviated as JJBA, is a shonen and seinen manga written and illustrated by Hirohiko Araki. JoJo's genre spans Action, Adventure, Supernatural, Thriller, Comedy, Tragedy, Mystery, Slice of Life, and Horror. 

Jonathan Joestar is the protagonist of Phantom Blood and the first JoJo of the JoJo's Bizarre Adventure series. The son of George Joestar I, Jonathan is an honest, kind, and gentle man whose life is fraught with tragedy after meeting his adopted older brother, Dio Brando. Jonathan becomes a Ripple user under the tutelage of Will Anthonio Zeppeli.

Joseph Joestar is the main protagonist of Part 2, the deuteragonist of Part 3 and an ally in Part 4. He is the second JoJo of the JoJo's Bizarre Adventure series. He is also the father of Josuke Higashikata and the second most recurring JoJo in the series after his grandson Jotaro Kujo being one of its most well-known characters. As the grandson of Jonathan Joestar, Joseph is a natural-born Ripple user and eventual Stand user, wielding the psychic photographic Stand, Hermit Purple.

Jotaro Kujo is the protagonist of Part 3 and also appears as a major character in Parts 4-6. Jotaro is the third and most recurring JoJo of the JoJo's Bizarre Adventure series. Wielding the incredibly powerful Star Platinum, Jotaro is the first JoJo introduced with a Stand and is among the most well-known characters of the series.
Jotaro's iconic cap is adorned near the center with a golden button and, on its left, a rectangular golden plate with a design of a flattened hand. It is torn at the back, appearing to merge with his hair. While they are first shown as being completely separate but slightly torn, later publications cause more uncertainty as to where his hat and his hair actually meet. According to Araki in an interview, he wanted readers to recognize Jotaro even from his back, and not just his forefront.

Josuke Higashikata is the protagonist of Diamond is Unbreakable and the fourth JoJo of the JoJo's Bizarre Adventure series. Josuke wields Crazy Diamond, a powerful Stand with the power to repair almost anything. 

Giorno Giovanna (ジョルノ・ジョバァーナ Joruno Jobāna) is the main protagonist of Part 5 and the fifth JoJo of the JoJo's Bizarre Adventure series. Giorno is the illegitimate son of DIO, conceived with Jonathan Joestar's stolen body. He is introduced as Haruno Shiobana. Giorno is a natural-born Stand User wielding the life-giving Gold Experience in battle.

Jolyne Cujoh is the main protagonist of Part 6 and the sixth JoJo of the JoJo's Bizarre Adventure series. Jolyne is the daughter of Jotaro Kujo. Initially an ordinary girl, while in prison, Jolyne awakens her string-based Stand, Stone Free. 
Like all other members of the Joestar family, Jolyne has a small star shaped birthmark on the rear of her left shoulder, close to her neck; and has tattooed on the top of her left forearm the image of butterfly wings superimposed with a downward-pointing dagger, which reappears on the chest of her outfits.

(from JoJo's Bizarre Encyclopedia)

The nickname "JoJo" is an abbreviation of the name of the Joestar family protagonists (Jonathan, Joseph, Jotaro, Josuke, Jolyne, Johnny, Jodio) in each of the eight manga story arcs whose adventures it chronicles.