Inline hooking is a method of intercepting calls to target functions,which is mainly used by antiviruses, sandboxes, and malware. The general idea is to redirect a function to our own, so that we can perform processing before and/or after the function does its; this could include: checking parameters, shimming, logging, spoofing returned data, and filtering calls. Rootkits tend to use hooks to modify data returned from system calls in order to hide their presence, whilst security software uses them to prevent/monitor potentially malicious operations.
The hooks are placed by directly modifying code within the target function (inline modification), usually by overwriting the first few bytes with a jump; this allows execution to be redirected before the function does any processing. Most hooking engines use a 32-bit relative jump (opcode 0xE9), which takes up 5 bytes of space.