Network security rests on three goals, called the CIA triad: confidentiality (only authorised parties can read data), integrity (data is not altered unnoticed) and availability (systems stay usable by legitimate users). Every security tool serves at least one of these. Encryption serves confidentiality. Hashing and message authentication codes serve integrity. Firewalls, redundancy and denial-of-service defences serve availability. Authentication — proving who you are — is a separate goal that supports all three: it is pointless to protect data if you cannot tell friend from attacker.
Two families of encryption exist. Symmetric-key cryptography uses one shared secret key for both encryption and decryption: fast, but the key must somehow reach both parties safely, and n users need n(n−1)/2 keys for private pairs. Asymmetric-key (public-key) cryptography uses a key pair: the public key encrypts, the private key decrypts, so anyone can send you a secret message without a prior shared secret. In practice, systems combine both: asymmetric cryptography exchanges a session key, and symmetric cryptography encrypts the bulk data, because symmetric ciphers are far faster.
Worked example — a Caesar cipher. Shift each letter by 3 positions: A becomes D, B becomes E, and so on, wrapping at Z. "HELLO" becomes "KHOOR": H→K, E→H, L→O, L→O, O→R. The Caesar cipher is trivially broken by trying all 25 shifts, which is why it is taught as history, not as protection. Real security needs keys long enough that brute force is infeasible and algorithms that survive public scrutiny: secrecy must live in the key, never in the algorithm's obscurity.