The "Black Magic Probe" is a debug probe for ARM Cortex microcontrollers, supporting SWD and JTAG. What sets it apart from other debug probes is that it directly supports the Remote Server Protocol of GDB (also used by LLDB and other debugging and development tools). This quashes the need for additional middleware, services or drivers, and thereby makes the software/hardware toolchain simpler, more portable and more reliable.

What you can do with the Black Magic Probe is:

Download code into Flash ROM or RAM of the microcontroller.
Step through code, look at a backtrace of the call stack.
Set up to six breakpoints in Flash ROM (as well as unlimited breakpoints in RAM).
Set up to four data watchpoints, to check for data reads or writes.
Inspect and modify variables, memory and registers.
Run the code and follow it with real-time tracing.
Profile your code, to gain insight in where it spends its cycles.

This book focuses on the original Black Magic Probe, but it also covers a few "open hardware" variants - such as the ctxLink probe, which offers truly remote debugging, due to its Wi-Fi interface and rechargeable battery.