There are certain modes of block cipher which operates very similar to stream cipher, but the main difference between the two is in case of block cipher the data is broken down into chunks (blocks) of 64 bit or 128 bits (depending upon algorithm used) while the stream cipher operates on individual bits of plain text.
So in case of block cipher of 64 bit block size if you want to encrypt a single character (8 bits) we need to add 56 bits (null bytes, also called pad) to complete one 64 bit block (8 + 56 = 64) for the algorithm to operate, while adding these 56 bits is not required in case of stream cipher as it can operate on individual bits.
A streambased cipher relies primarily on substitution (replacing bits).
The keystream must be strong enough to not be easily guessed or predictable. In time, the keystream will repeat, and that period (or length of the repeating segment of the keystream) must be long enough to be difficult to calculate. If a keystream is too short, then it is susceptible to frequency analysis or other language specific attacks.