As the question says, what is still life photography?
Are there any 'rules' as to what it can feature? i.e., what sort of subjects?
Does it need to be 'artistic' or to have a particular style?
To quote from Wikipedia:
A still life (plural still lifes) is a work of art depicting mostly inanimate subject matter, typically commonplace objects which may be either natural (food, flowers, dead animals, plants, rocks, or shells) or man-made (drinking glasses, books, vases, jewelry, coins, pipes, and so on).
So, you can think of it as "thing photography". It can overlap with macro and product photography, naturally.
However, there is no set style or intent to still life photography, the category is determined solely by content. That doesn't mean it can't be art. But it can also be hackery, depends on the artist and the audience. Just understand that, as with painting, still life isn't necessarily just about the thing that's pictured (see: Weston's Pepper No. 30).
I like the definition from the Coursera Course "Photography Techniques: Light, Content, and Sharing". I think this text was written by one of the Instructors, Prof Mark Sullivan:
In general, the genre of "still life" involves inanimate objects placed into a composed order, in a context, with a background. Many different kinds of objects can be used - a random collection of objects taken from everyday life - a collection of different colored feathers, books next to a watch on a table top, and so forth.
In creating a still life, composition becomes even more important, as the subject matter, by itself, may not be striking. In a sense, everything depends directly on composition: Leading lines, geometric relationships, the perspective implications generated by the objects (including other principles like the grid of thirds, the use of negative space, etc.), the positioning of shadow, the use of contracting and complementary textures, as well as the hardness or softness of chosen lighting, quite directly determine the effectiveness of the image.
Still life images can effectively use either hard, direct lighting or soft, indirect lighting. Which will be appropriate depends on the objects, the intended order, and the desired emotional ambience - hard lighting lends itself to certain kinds of dramatic effects, or austerity and roughness; soft, diffuse lighting can be more suggestive, elegant, smooth and so forth. Ultimately, the choice of lighting is crucial to creating the photographer's intended concept, and cannot be evaluated independently of that concept.
According to this description, I'd say, Still Life Photography sits in between "ordinary" photography and photomontage. Worth pursuing when you think your personal artistic development evolves towards abstraction and/or explicit storytelling.
Usually table top photography, usually intended as artistic. Still life is things that don't move, like fruit bowls or pottery, could be food, etc.