Joi appears generally as a flawlessly beautiful petite young woman. The specifics of her features, such as eye color, hair color, wardrobe, and likely some aspects of her body are variable from instance to instance. She has a variety of vocal intonations and can adopt accents. She is a holographic projection and requires some kind of projecting device to manifest to the viewer/user, whether a ceiling-mounted projector for a single room, a building-scale advertising projector, or a portable "Emanator" device that allows Joi to travel with her user.
Joi's hardware is able to extensively sense the environment it is in and translates that into data for the artificial intelligence to "experience" life along with the user. The hardware senses whether Joi is projected or not.
Joi's primary role is to communicate with other entities, human, replicant or otherwise, in a manner representing the user's mate or companion. She records data "memories" as she runs, allowing her "personality" to change over time.
Joi's program and memories reside on a main console, which for personal units is located in the home, as well as on portable Emanators or other linked devices. Joi hardware has the ability to utilize communications networks to contact other entities on its own initiative, and can be used to monitor or track Joi users.
Was Joi "real"?
Similar to the "Is Deckard a replicant?" question, Joi's existence in the film brings up similar ideas of what is considered real and why it matters.
Joi is real
Several points of view and details in the film indicate Joi to be as real as anyone else in the film. She seems to decide on her own to aid K in evading the police and Wallace Corporation by placing herself in the eminator and breaking the antenna. She claims she wants to be real for K.
From the point of view of K, Joi is for him. She satisfies his emotional needs and more. Is Joi any different than a person shaping themselves to be what one wants them to be?
Joi is not real
Joi is not only just one; she can be purchased by any consumer. You can customize Joi in countless ways, and make her who you want her to be. Everything you want to hear and see implies that K's Joi might be following her programming so well that she is simply being what he wants her to be: real. Not because she has grown beyond what she was made for, but because she is doing precisely what she was made for (even calling Joi "she" in this context would be erroneous, where "it" may fit better").
K's encounter with the giant purple Joi ad establishes that Joi has responses that are universal for her product. "What a day" being one of them, she also calls K a "good Joe".