The TIB is officially undocumented for Windows 9x. The Windows NT series DDK includes a struct NT_TIB in winnt.h that documents the subsystem independent part. Yet so many Win32 programs use undocumented fields so it is effectively a part of the API.

The TIB can be used to get a lot of information on the process without calling win32 API. Examples include emulating GetLastError(), GetVersion(). Through the pointer to the PEB one can obtain access to the import tables (IAT), process startup arguments, image name, etc.

[edit] Accessing the TIB

The TIB can be accessed as an offset of segment register FS.

It is common to not access TIB fields by offset from FS:[0], but rather first getting a the linear self-refernecing pointer to the stored at FS:[0x18]. That pointer is used in means of pointer arithmetics or cast to a struct pointer.

Example in C inlined-assembly for 32-bit x86:

// gcc (AT&T-style inline assembly).
void *getTIB()
{
    void *pTib;
    __asm__("movl %%fs:0x18, %%eax\n\t"
            "movl %%eax, %0"
            : "=rm" (pTib) : : "%eax");
    return pTib;
}

// Microsoft C
void *getTib()
{
    void *pTib;
    __asm {
        mov EAX, FS:[18h]
        mov [pTib], EAX
    }
    return pTib;
}

[edit] Contents of the TIB