"Arrays: are created dynamically, stored as B-trees, are sparse (i.e. use almost no space for missing nodes), can use any number of subscripts, and subscripts can be strings or numeric (including floating point). Arrays are always automatically stored in sorted order, so there is never any occasion to sort, pack, reorder, or otherwise reorganize the database. Built in functions such as $DATA, $ORDER, $NEXT(deprecated) and $QUERY functions provide efficient examination and traversal of the fundamental array structure, on disk or in memory.

Local arrays: variable names not beginning with caret (i.e. "^") are stored in memory by process, are private to the creating process, expire when the creating process terminates. The available storage depends on implementation. For those implementations using partitions, it is limited to the partition size, (A small patition might be 32K). For other implementations, it may be several megabytes.

Global arrays: ^abc, ^def. These are stored on disk, are available to all processes, and are persistent when the creating process terminates. Very large globals (e.g., hundreds of gigabytes) are practical and efficient in most implementations. This is MUMPS' main "database" mechanism. It is used instead of calling on the operating system to create, write, and read files."

http://en.wikipedia.org/wiki/MUMPS#Summary_of_key_language_f...

It might be a punchline, but I'm writing code in it right now.