What characterizes a 'set' in Python?

A 'set' in Python is characterized as an unordered collection of unique items. This means that when you create a set, the elements are not stored in a specific sequence, and any duplicate values are automatically removed. The uniqueness of items is a defining feature, as it allows sets to be used in operations where distinct values are critical, such as mathematical set operations (union, intersection, difference).

Additionally, because sets do not allow duplicates, they are ideal for situations where you need to ensure that each element is represented only once. The unordered nature of sets also means they do not support indexing, slicing, or other sequence-like behavior, differentiating them from lists or tuples.

This understanding highlights the properties of sets in Python, making option B the correct answer in the context of how sets function within the language.