What's the difference between a breakpoint and a watchpoint?

A breakpoint stops execution when the program reaches a specific code location, when the program counter hits a given line or function address, so it answers "stop when control flow gets here," which is what you use to inspect state at a particular point in the logic. A watchpoint, also called a data breakpoint, stops when a specific memory location is accessed or changed, when a variable is written (or read, or either), so it answers "stop when this data is touched, regardless of which code does it." The distinction matters because they solve different problems: a breakpoint is for control-flow questions ("what's the state when we enter this function?"), while a watchpoint is for data questions, most powerfully "what code is corrupting this variable?", you set a watchpoint on the variable's address and the debugger halts at the exact instruction that writes it, then a backtrace reveals the culprit even if you had no idea where the bad write lived. So breakpoint = PC-reaches-address, watchpoint = data-address-is-accessed, and the watchpoint is the specialized tool for memory-corruption bugs that a breakpoint can't find because you don't know where to put it.