J-Link

A probe is the little box between your USB port and the board's debug header. It turns "halt here" and "read this address" into signals on JTAG or SWD. Germany's SEGGER has made the J-Link since the early 2000s, and it has become something of an industry default, mostly because it supports nearly every ARM core plus several other architectures, and because it writes flash quickly.

Its best-loved feature is RTT (Real Time Transfer): firmware writes log text into a ring buffer in RAM, and the probe reads it out while the core keeps running. That gives you printf debugging with no UART and almost no latency. The competition is ST-Link, CMSIS-DAP and, on the software side, OpenOCD.