Linux Filesystem Hierarchy

1.9. /lib
The /lib directory contains kernel modules and those shared library images (the C programming code library)
needed to boot the system and run the commands in the root filesystem, ie. by binaries in /bin and /sbin.
Libraries are readily identifiable through their filename extension of *.so. Windows equivalent to a shared
library would be a DLL (dynamically linked library) file. They are essential for basic system functionality.
Kernel modules (drivers) are in the subdirectory /lib/modules/'kernel−version'. To ensure proper module
compilation you should ensure that /lib/modules/'kernel−version'/kernel/build points to
/usr/src/'kernel−version' or ensure that the Makefile knows where the kernel source itself are located.
/lib/'machine−architecture'
Contains platform/architecture dependent libraries.
/lib/iptables
iptables shared library files.
/lib/kbd
Contains various keymaps.
/lib/modules/'kernel−version'
The home of all the kernel modules. The organisation of files here is reasonably clear so no requires
no elaboration.
/lib/modules/'kernel−version'/isapnpmap.dep
has details on ISA based cards, the modules that they require and various other attributes.
/lib/modules/'kernel−version'/modules.dep
lists all modules dependencies. This file can be updated using the depmod command.
/lib/modules/'kernel−version'/pcimap
is the PCI equivalent of the /lib/modules/'kernel−version'/isapnpmap.dep file.
/lib/modules/'kernel−version'/usbmap
is the USB equivalent of the /lib/modules/'kernel−version'/isapnpmap.dep file.
Linux Filesystem Hierarchy
Chapter 1. Linux Filesystem Hierarchy
50

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/lib/oss
All OSS (Open Sound System) files are installed here by default.
/lib/security
PAM library files.
The FSSTND states that the /lib directory contains those shared library
images needed to boot the system and run the commands in the root filesystem,
ie. by binaries in /bin and /sbin.
Shared libraries that are only necessary for binaries in /usr (such as any 
X Window binaries) must not be in /lib. Only the shared libraries required
to run binaries in /bin and /sbin may be here. In particular, the library 
libm.so.* may also be placed in /usr/lib if it is not required by anything
in /bin or /sbin.
At least one of each of the following filename patterns are required (they 
may be files, or symbolic links):
libc.so.* The dynamically−linked C library (optional)
ld*       The execution time linker/loader (optional)
If a C preprocessor is installed, /lib/cpp must be a reference to it, for
historical reasons. The usual placement of this binary is /usr/bin/cpp.
The following directories, or symbolic links to directories, must be in 
/lib, if the corresponding subsystem is installed:
modules   Loadable kernel modules (optional)
/lib : Alternate format essential shared libraries (optional)
There may be one or more variants of the /lib directory on systems which
support more than one binary format requiring separate libraries.
This is commonly used for 64−bit or 32−bit support on systems which support
multiple binary formats, but require libraries of the same name. In this 
case, /lib32 and /lib64 might be the library directories, and /lib a symlink
to one of them.
If one or more of these directories exist, the requirements for their contents
are the same as the normal /lib directory, except that /lib/cpp is 
not required.
/lib/cpp is still permitted: this allows the case where /lib and 
/lib are the same (one is a symbolic link to the other).