Sculptures in the round have details on all sides of the sculpture. In one sense, this tends to make these works more 'life-like' or 'real'. Here is a sculpture by Michelangelo. A sculpture in the round cannot also be in relief. Even if there is not access to the back of a sculpture, if that work is finished on all sides** then it is in the round. Here is an example of a sculpture within a niche. Note that the sculpture is finished on all sides.

**This does not mean that the sculpture was not completed, but rather that it was completed and finished on all sides. Relief sculptures are typically meant to be viewed from one side. Sculptures in the round are meant to be viewed from more angles around the work.