PKCS #1
PKCS #1 is the standard for RSA operations, and its padding has caused more confusion than RSA itself.
RSA Laboratories published PKCS #1 in the early 1990s as a common format for RSA encryption and signatures. Version 1.5 fills the message with random padding, and that padding was the basis of Bleichenbacher's 1998 attack. A server that revealed whether the padding was valid could be broken with roughly a million queries. Version 2.x, with OAEP and PSS, is what you should use now, but v1.5 lives on in older protocols.
The standard is consolidated in RFC 8017 from 2016, and that is the version to read today.