What is the difference between a 'cipher' and a 'mode of operation'? What is the difference between the term cipher (a name like RIJNDAEL) and mode of operation (like ECB)?

Aren't these both terms for the encryption/decryption technique?
A mode of operation is an explicit method by which we use a block cipher (eg AES) to do more than just encrypt one block of data. For example, it may allow us to encrypt multiple blocks of data (eg ECB,CBC etc), provide us with some authenticated encryption (eg GCM) or a method for encrypting disc storage (eg XTS).



Rijndael,DES etc are block ciphers. That means they provide a method for encrypting a fixed amount of data (in the case of AES, they encrypt blocks on 128 bits).

In practice, the amount of data you want to encrypt is often much larger than the block size. So, we either need to create more and more block ciphers of almost every size, or we come up with a way of using our block cipher to encrypt larger amounts of data.  This method is called a mode of operation.

To use the most simple case as an example, consider AES in ECB mode. ECB mode tells us that we need split our plaintext string into sections of 128 bits (possibly after some padding scheme). Then, each section is encrypted using AES, and the resultant encryptions are then concatenated to form our output ciphertext.
AES simply performs encryptions of the plaintext blocks we give it, whereas ECB allows us to encrypt arbitrarily large amounts of data.



For descriptions of many different modes of operation and a discussion of their merits/weaknesses, you might be interested in this paper by Rogaway, and section 2.3 provides his formalisation of a mode of operation.