SB-Link is a connector on the mainboard (for example 6LTM2) which is especially for use with a Creative Labs PCI soundcard. The SB-link guides signals from the ISA bus to the PCI soundcard through a cable which comes with the PCI soundcard. This is necessary because some DOS based games address the ISA bus directly. In this way compatibility with these games is guaranteed. The official explanation of SB-Link is as follows:
SB-LINK combines Intel's PC-PCI and "Serialized IRQ" protocols. These technologies can be found in Intel's TX, LX and newer core logic chip sets. This technology provides the DMA and IRQ signals present in ISA Bus today, but not available on the PCI Bus. The SB-LINK serves as a bridge between the motherboard and PCI sound card to deliver Sound Blaster 16 compatibility for real-mode DOS games, a widely accepted audio standard in Multimedia Personal Computers. SB-LINK, endorsed by leading motherboard suppliers, is becoming a standard audio connector on new motherboards. With SB-LINK, sound card users can look forward to the best gaming experience ever.