Xlib is an X Window System protocol client library written in the C programming language. It contains functions for interacting with an X server. These functions allow programmers to write programs without knowing the details of the protocol. Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide widget toolkits:

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Functions
Display *XOpenDisplay(display_name)
Window XCreateSimpleWindow(display, parent, x, y, width, height, border_width, depth, class, visual, valuemask, attributes)
Window XCreateWindow(display, parent, x, y, width, height, border_width, depth, class, visual, valuemask, attributes)
XQueryBestTile(display, which_screen, width, height, width_return, height_return)
XLib Function - XLookupString

Structures
XEvent
XButtonEvent
XKeyEvent
XMotionEvent

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Resources
Tronche - The X Window system
Wikibooks - X Window Programming/XLib
X.Org Foundation wiki
Volume One: Xlib Programming Manual