Simulink Desktop Real-Time (SLDRT) kernel is installed with the desktop OS (Mac or Windows). It is limited to using a single cpu core. Simulink Real-Time (SLRT) kernel runs on a dedicated computer so it will be able to leverage multicore platforms.
They are both able to use external mode type features (tunable parameters etc). The I/O offerings are slightly different (SLRT supports use of FPGAs).