An arrays contains an ordered set of elements while a multiset stores an unordered set of elements.
A multiset can contain the same element multiple times.
These collection types are used to store a homogenous collection of elements (that is: every element has the same data type). Thus, with multiset, their underlying data type needs to be specified, for example with integer multiset.
The number of elements in an array can be determined with the cardinality function.
Starting with SQL:2011, the maximum number of elements that can be stored can be set wtih array_max_cardinality.
SQL:2011 also introduced trim_array which can be used to remove elements from the end of an array.