An alist is a simple data structure that holds key-value pairs in a linked list. When a key is looked up, the list is searched to find it. The time it takes is proportional to the length of the list, or the number of entries. A hash-table is a more complex data structure that holds key-value pairs in a set of "hash buckets". When a key is looked up, it is first "hashed" to find the correct bucket,