A singly linked list is a data structure consisting of nodes where each node stores an element and a link to the next node. The document describes algorithms for inserting nodes at the head, tail, and between two nodes of a singly linked list. It also describes doubly linked lists which allow traversing the list in both directions by including a link to the previous node as well as the next node in each node. Linked lists are useful when the number of elements is unknown or varies, and for operations like inserting or removing elements in the middle of the list without shifting other elements.