What is the difference between the stream ciphers and the block ciphers??
Is the difference the time complexity??
At the block ciphers the message is cut into parts of $n$ characters. If we have for example the word "anna" in two parts, then the cryptographized message is the same at each part??
If we have a block cipher, then this is defined on blocks of some number of bytes $n$, typically 8 bytes (for older ciphers, like DES, Blowfish, CAST5), or 16 bytes (for modern ones like AES, Twofish, etc.). There is a key (of some size $k$), and then for every fixed key, a block cipher is a permutation between all blocks of size $n$ (a good one should be indistinguishable, computationally speaking, from a random permutation).
In order to be able to process arbitrary streams of data, we have to cut up the stream into chunks of length $n$, and we have to take into account that we can have a partial block at the end (using forms of padding, or filling out of data). Also, naively encrypting each block separately and independently of the others leaks information, because the same input block will later be encrypted to the same output block. So usually other so-called "modes of operation" are used (like CBC) to randomise the different blocks.
A stream cipher is an algorithm, also controlled by some key of lenght $k$, that typically processes data in a stream directly (no special mode of operation, typically), either bitwise or per byte. If it's an autonomous stream cipher, as most are, then it produces a key stream (bits or bytes) that is xor-ed with the plain data. So for such stream ciphers, the plain data is not used in the algorithm directly, but a key and possibly some extra random data (an IV) is used to produce some internal state that is updated for every bit or byte that we produce as keystream output. E.g. look up typical stream ciphers like A5/1 or RC4 to see how this works in concrete examples.
Typically, stream ciphers were used in hardware applications (like GSM, Bluetooth, Wifi), because they were generally faster in producing output than blockciphers. With modern block ciphers having hardware support, this is less true today, and except for those stream ciphers that are supported in standards and RC4 (which is still popular in TLS, e.g.), they're not that heavily used any more. Most modern standards have some form of blockcipher mode (even counter mode, which makes a block cipher into a stream cipher, essentially); dedicated stream ciphers are just not that popular.