Mode of operation
A mode of operation decides how a block cipher is applied when the message is longer than one block.
A block cipher handles one block at a time, so something has to decide how the blocks relate, if they relate at all. NIST SP 800-38A, published in 2001, describes the five classic modes: ECB, CBC, CFB, OFB and CTR. ECB leaks patterns in the data, and it should be the first thing struck from every cryptography course.
Authentication was added later, and GCM and CCM are the usual choices today. The block cipher itself is rarely the weak point. More often it is the mode, the initialization vector or the padding around it.