Set theory is the branch of mathematics that deals with sets, that is unordered collections of distinct things (called 'elements'). Sets are usually denoted by enclosing their list of elements in curly brackets, e.g. {dog, cat, budgerigar}. The most common operations in set theory are 'union' (the set containing everything that is in two sets) and the 'intersection' (set of common elements), usually written ∪ and ∩ respectively. For example, if set A is {dog, cat, budgerigar} and set B is {aligator, cat, dog}, then A∪B is {cat, budgerigar, dog, aligator} and A∩B is {dog, cat} (recall order doesn't matter).