The increasing use of digital communication systems, cloud computing, and internet services has created major concerns regarding data security and privacy. Cryptography plays an important role in protecting sensitive information from unauthorized access and cyber threats. This survey paper presents an overview of major cryptographic techniques used in secure communication systems. The study discusses classical cryptography, symmetric key cryptography, asymmetric key cryptography, and hashing techniques. Important algorithms such as DES, AES, RSA, ECC, and SHA are analyzed based on security, performance, and practical applications. The paper also highlights the use of cryptography in wireless networks, cloud computing, Internet of Things (IoT), blockchain technology, and banking systems. In addition, major challenges including quantum computing threats, key management issues, and computational complexity are discussed. The study concludes that advanced and lightweight cryptographic methods are essential for improving security in modern digital communication environments