The ULINK2 debug unit connects your PC's USB port to your target system (via a JTAG, Cortex Debug, or OCDS connector). It allows you to program and debug your applications on target hardware.
ULINK2, together with Arm Keil MDK, provides excellent debug capabilities for Cortex-M devices. You can control the processor, set breakpoints, and read/write memory contents, all while the processor is running at full speed.