Operation modes of block ciphers how are used? I am studying about different operation modes of block ciphers my question isn't something specific to that, but is how these operation modes are used in cryptography?

I have this picture here (OFB-mode):


(source)

I would like to know how these operations are combined with a block cipher algorithm? For example we have the DES encryption, while the DES has it's own encryption method where does these operations intersect with the algorithm? I mean when these operations are used in the actual DES algorithm? And why we need them if we know that the DES itself is an encryption method?
DES would be the box in your diagram that's labeled "Block Cipher Encryption", and it participates in the encryption mode you show in two ways: the output is XORed with the plaintext, and the output is used as the input to the next block's use of DES (the second "Block Cipher Encryption").

In this mode, you're using DES to generate a sequence of random blocks based on the IV and key, so it's operating as a stream cipher (which means you must NEVER use the same IV and key more than once, in other words you need to use a new unique IV for each new set of message blocks as long as the key stays the same). OFB turns a block cipher such as DES into a stream cipher.

The requirements for an IV can be different for different modes, as are the consequences of re-using an IV with the same key.

Using the "raw" encryption algorithm on each block is known as ECB, Electronic Code Book.  The reason you don't normally use ECB is that any time you encrypt the same block with the same key, you'll get the same output.  This can easily yield a lot of information about the message, something that you don't want.  There's a nice image showing how much information can leak using ECB mode in the Wiki link above.

The different chaining modes have different requirements, before using one you need to understand what the pitfalls, advantages and drawbacks are.  In particular, most of the modes need to have some form of message authentication to prevent an attacker from changing parts of the encoded data.  The way to do this with the common chaining modes is called a MAC, a Message Authentication Code.  An example of a MAC is called HMAC (Hash-based message authentication code)(which, itself, is a mode of operation using some underlying hashing algorithm).

The problem with not using any message authentication (which is not the same as authenticating WHO you're talking to) is that an attacker can do many nasty things, depending on the chaining mode.  This can lead to them being able to change messages or determine part or all of what's in a message.