What do you understand by cryptography?

Cryptography is the art and science achieving security by encoding message to make them non readable.
Cryptographic systems are characterised along three independent dimensions

1 The type of operations used for Transforming plaintext to Chiphertext:
All encryption algorithm are based on two general principals solution, in which each element in the plaintext (bit, letter, group of bits or letters) is mapped in to another element, and transposition, in which elements in the plain text are rearranged. The fundamental requirement is that no information be lost (that is, that all operation are reversible). Most systems, referred to as product systems, involve multiple stages of substitutions and transpositions.

 2 The number of keys used:
If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key pr conventional encryption. If the sender and receiver each uses a different key, the system is referred to as asymmetric, two-key, or public-key encryption.

3 The way in which the plaintext is processed:
A block cipher processes the input one block of elements at a time, producing an output block for each input block. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.