Electronic Codebook (ECB) is the simplest Block cipher mode of operation.
Split input into blocks, each block is independently encrypted with the same key.
The weakness is that same blocks would produce the same results. Thus, ECB does not hide data patterns.
ECB is also susceptible to Replay attack because each block is decrypted in exactly the same way.
It is not recommended to use ECB in cryptographic protocols at all.
Inability to hide data patterns is very striking on images with large monotonic colors. Input:
Encrypted with ECB:
(while every color is encrypted, the overall image pattern is very visible)
Compare this with CBC (or other mode):
(Note that if image looks random to the eye, that does not mean it is securely encrypted)