What is <x>.dll used for?

"Check this site for Windows 8, Windows 7, or Windows XP

Welcome to DLL information site for Windows XP/7/8.
This site was built by scanning all DLL files located in system32 directory of Windows XP with SP3 and creating (with automatic script) a Web page for each DLL with all found infromation.

Each DLL information page includes:

Version information - product name, company, file desctiption, and so on.
DLL popularity - Shows you how many DLLs are statically linked to this file.
List of files that are statically linked to the specified file. Displayed only when the number of files in the list is 10 or less.
Sections information - Shows you the code and data sections in the DLL.
Resources information - Displays a summary of resources stored in the DLL (icons, bitmaps, cursors, dialog-boxes, and so on)
Icons Thumbnail - A Thumbnail with all icons stored in the DLL.
Cursors Thumbnail - A Thumbnail with all cursors stored in the DLL.
Strings information - Displays a list of strings stored in this DLL (Currently the list is limited to 200 strings)
Diaogbox information - Displays a list of dialog-boxes captions in this DLL (Currently the list is limited to 200 dialog-boxes)
Static Linking - Displays the list of DLL files that are statically linked to the DLL. When a DLL is loaded the DLL in this Static Linking list are also loaded with it.
Exports/Imports List - A list of all imported and exported functions.

Looking in the Web page of specific DLL can help to learn what is this DLL and how it's linked with the other DLL files on Windows XP operating system."