There are no hard and fast guidelines in length for either short stories or novellas but the most important difference between these literary forms is length. In his "Philosophy of Composition," Edgar Allen Poe wrote, "that there is a distinct limit, as regards length, to all works of literary art --- the limit of a single sitting..." With his next words, Poe recognized the falseness of this statement but that is a good length for a short story, while a novella can require two or three sittings. A long short story may easily be read as a short novella and a short novella as a long short story.
Short stories are almost never sold in stand-alone form. They are almost always sold as part of an anthology, whether the short stories are by other writers or the same writer, or as a feature in a newspaper or magazine. Novellas can be published in collections, particularly if they are short or there are only a few of them in the anthology, but they are more usually published as complete books.
The shorter the work, the less room there is for theme, characters and plot and character development. A short story should be one scene with a bare minimum of characters---no more than three, and two is better. A novella might permit half a dozen characters if each is very important and several scenes, perhaps even several chapters of two or three scenes each. However, according to Timothy Antrim of "The Daily Beast", the novella "03" is a single paragraph that is 85 pages long.
The length of a novel permits the reader to follow marriages, entire lives and even wars from beginning to end, with enormous latitude for both ambiguity and discursiveness, as well as resolution. The short story focuses entirely on a single important event; there is room for neither ambiguity nor discursiveness, and it almost requires a decisive resolution. A novella is different. Its short length requires it to be nearly as cohesive as a short story in its focus on a few characters. Yet because it is longer than a short story, as Antrim noted, the novella lends itself to both ambiguity and eccentricity.