the data is separated in larger chunks and the cipher encrypts a block at a time (usually a block size is at least 64 bits and can be up to 128 or 256 bits)
consists of a stream of data and the cipher can encrypt small chunks at a time (usually 1 byte at a time)
Security implications
never use the same keystream multiple times
key length is important
Usage
Stream ciphers are often used for their speed and simplicity of implementation in hardware, and in applications where plaintext comes in quantities of unknowable length like a secure wireless connection
Another advantage of stream ciphers in military cryptography is that the cipher stream can be generated in a separate box that is subject to strict security measures and fed to other devices such as a radio set, which will perform the xor operation as part of their function