One of Jojo's Bizarre Adventure's most unique and recognizable aspects is the Stand, a manifestation of the user's soul that fights (and "stands") alongside them. While Stands were only introduced in part 3, Stardust Crusaders, they quickly became the norm, and all subsequent parts have focused heavily on Stands, Stand Users, and their origins. Stands can be quite complex, however, so it's good to have a clear idea of just what a Stand entails.
How Does One Get a Stand?
Right from the start, Stands can manifest in a variety of different ways. Jotaro's Stand, Star Platinum, begins to manifest on its own, causing Jotaro to think he's being haunted by some sort of demon. Similarly, Jotaro's mother, Holly, starts manifesting a Stand that seemingly exceeds her body's capabilities, threatening her life and kickstarting the plot. Stands may manifest at any point in life, and typically reflect the user's age and size to some degree. One doesn't even need to be human to manifest a Stand; over time, Orangutans, dogs, birds, rats, cats, a sword, and even certain locations have all been shown to possess Stands.
Not everyone seems to have the potential to become a Stand user, however. Another way of invoking a Stand is through the use of an artifact called the Stand Arrow, which can draw out the power of people struck by it so long as they survive. In part 4, Koichi's Stand Reverb comes about after he is struck by the arrow, for example. While most Stands are silent, there are occasionally Stands that can communicate independently from their user to the point of carrying on conversations or arguments, like Koichi and Reverb.
Stand Variants
Stands also come in a variety of different types. Close range Stands are, as the name suggests, most effective at short distances, such as Star Platinum. They are extremely powerful but usually can't venture far from their user. Long-distance Stands are the opposite--often weaker, but capable of acting on the user's behalf from potentially miles away. Relatively few protagonist characters have had long-distance Stands, but one example would be Narancia's Little Bomber from part 5, which took the form of a remote control plane that could attack and patrol from the air. Stands that operate on their own, without input from the user, also exist. These are usually called automatic-type Stands and can perform certain simple actions entirely on their own. Their range may not be relevant; the Oingo Boingo brothers' Thoth Stand predicts the future, for example, and so isn't restricted by distance at all.
Stands can have many different appearances as well. Most Stands are humanoid in appearance and even wear armor or carry weapons. Humanoid stands are typically shown hovering over the user or standing off to their side. They can be very human-looking, with only unusual skin tones or hair to tell them apart, or monstrous, only humanoid in the vaguest sense. Avdol's Magician Red, for example, takes on the appearance of a bird-headed man, while Polnareff's Silver Chariot could easily be mistaken for a man in armor.
Some Stands look more like creatures, and may have more than one "body," as is the case for Shigechi's Harvest Stand in part 4, which takes the form of large bug-like beings that can retrieve items for him. Stands don't have to resemble living things at all, though. The aforementioned Thoth Stand takes the form of a book, while part 3 antagonist Hol Horse's Emperor Stand takes the shape of a gun which can be summoned into his hand at will. Stands can also lack a visible body entirely, only recognizable by their effect and making them quite difficult to defeat. Many fans believe that all Stands have a humanoid form, but not all users are skilled enough to manifest it. There's very little evidence to support or disprove this theory, but some Stands which initially were not humanoid have been portrayed as such in later appearances.
Stands are also not always bound by a single form. Some, like Koichi's Reverb, evolve over time, passing through different stages referred to as "Acts." These evolutions often come with significant growth of power and new abilities. If someone is already a Stand user and is struck by the Stand Arrow, their Stand may transform to a completely new form, called a Requiem form, which played a major role in Part 5, Golden Wind. Requiem Stands are some of the most powerful on display in the series, altering time and space in incredible ways. Both of these conditions are quite rare, however.
Stand Differences in the Alternate Continuity
It's important to note that after part 6, Stone Ocean, stories take place in a different universe with different continuity. Stands in the new continuity operate slightly differently, so not everything from previous installments is carried forward. The Stand Arrow, for example, has not been shown to exist in the universe where parts 7 and 8 take place. Instead, most Stands have emerged from people passing through dangerous areas known as Devil's Palms, which form where the remains of the Saint's Corpse (implied to be Jesus) are located. The Saint's Corpse is also capable of providing a "Blessing," which can improve the power of a Stand further. Since all nine parts of the Saint's Corpse were originally located in America, most Stand users seem to be American in this universe, including Johnny Joestar. The corpse was temporarily relocated to Japan, which in part 8 led to the Higashikata family and others around Morioh developing Stands as well.
Stand users are said to be drawn to one another by fate; when users meet it often results in a battle, but could just as easily introduce new allies. Villains who use Stands to achieve their aims tend to attract the attention of the protagonists in this way, even if the protagonists might otherwise have no knowledge or awareness of the user, as was the case with Yoshikage Kira. The fact that any Stand user can see all Stands in the area makes it particularly easy for them to identify each other, at least in the case of close range stands. While Stands in Jojo's Bizarre Adventure have gotten more complex over time, and will almost certainly continue to do so in the upcoming part 9, the basics of how Stands work have remained thankfully consistent.