DNSBL stands for Domain Name System-Based Blacklist, which is a database containing IP addresses or domain names known to be sources of spam or other malicious activities. RBL, or Real-time Blackhole List, is a term that is often used interchangeably with DNSBL. However, while all RBLs are DNSBLs, not all DNSBLs are RBLs. The distinction lies in their origin: RBL was a specific blacklist created to combat email spam, while DNSBL is a more general term that encompasses various blacklists, including those not specifically designed for email filtering.