The document introduces Return Oriented Programming (ROP) and its core concepts. It discusses how Data Execution Prevention (DEP) prevents execution of injected shellcode by marking the stack and heap as non-executable. ROP overcomes this by chaining together small snippets of existing code (called "gadgets") in libraries and binaries to achieve arbitrary code execution. This is done by creating fake stack frames and controlling the instruction pointer (EIP) to return to addresses of gadgets. An example demonstrates overflowing a function and making it return to another function by placing a fake stack frame.