The difference should be clearer when you look at the behaviour of each when you pass it an Iterable (for example a List):
Observable.just(someList) will give you 1 emission – a List.
Observable.from(someList) will give you N emissions – each item in the list.
The ability to pass multiple values to just is a convenience feature; the following are functionally the same:
Observable.just(1, 2, 3);
Observable.from(1, 2, 3);