What is a block cipher? I have read about block ciphers, but I do not understand the topic completely.

Is a block cipher a way to send data or a way to encrypt data?

How is the data divided into multiply blocks before it is sent? Is the data partitioned before the encryption process or afterwards? 

How long are the blocks? Can I choose any block size?
A block cipher is any cipher that works(encrypts, decrypts) on data in blocks (more than just one bit or one byte). A stream cipher is a cipher that works on the data one bit or byte at a time.

For example, DES is a block cipher because it works on the data in 64 bit chunks. This means that when you want to encrypt something with DES, you must first break it into 64 bit chunks, and encrypt each chunk with DES. Same with decryption.

An example of a stream cipher is RC4. If you want to encrypt using RC4, you need to encrypt each byte individually. There is no need to create chunks of that data, because RC4 works on the data one bit at a time.

You cannot choose a blocksize, because it's generally dictated by the algorithm you choose. How you send the data is not related to how you encrypt the data. That means a block cipher only says that you need to encrypt in a block, but says nothing about how you should transmit the data.