An intensive study of the knapsack problem is provided as it proves its effectiveness and success within the computer science field. The knapsack problem (KP) is NP-hard problem and finding the optimal solution to the larger one in practice is impossible. This paper provides a focused literature review of the Knapsack problem. In addition; it discusses distinct variations of KP, investigates its numerous applications in several areas, and presents different algorithms for solving the knapsack problems.