This document discusses approximation algorithms and introduces several combinatorial optimization problems. It begins by explaining that approximation algorithms are needed to find near-optimal solutions for problems that cannot be solved in polynomial time, such as set cover and bin packing. It then provides examples of problems that are in P, NP, and NP-complete. Several techniques for designing approximation algorithms are outlined, including greedy algorithms, linear programming, and semidefinite programming. Specific NP-complete problems like vertex cover, set cover, and independent set are introduced and approximations algorithms with performance guarantees are provided for set cover and vertex cover.