What is the difference between singly and doubly linked lists?

A singly linked list has nodes with data and a pointer to the next node only, allowing traversal in one direction. A doubly linked list has nodes with pointers to both next and previous nodes, enabling bidirectional traversal. Doubly linked lists use more memory but allow O(1) deletion when you have a reference to the node, while singly linked lists require O(n) to find the previous node.