Friends,
I'am new to 3d graphics programming. Please tell me what is a game engine. What is its role in game development. Is it a collection of useful functions ???
Please shed some light. 
hum… :>
well… if u make a game u need media (sound, graphics, models), levels (or maps) and an engine wich displays the stuff.
the engine is basically the heart of the game. it’s needed to work. it determines wich stuff has to be shown and how… mixin sound and well basically everything wich is called then “the game” 
eg id-soft sells their engines to other companies for a lot of $ (250.000$ for the q3 engine). the companies take it… change everything they wanna… add new media and a bunch of levels and they r done. a new game is born (alice, rtcw, heavy metal fakk2… etc).
an engine can be everything from a main loop wich is 100 lines long to giant programms build by several ppl several years long with a million lines of code.
the q3 engine does a lot of stuff. loading media from pk3s (zip), a console, keybindings, soundmixing, physics, loading a bsp tree… render only the stuff wich is visible from the current pvs(pov), handling special fx wich r written in a strange shader language, a scripting language for controls, bot logic (ai), game logic… and one impressive thing r the "qvm"s (quake virtual machine - u can change almost everything this way) wich r loaded by the engine they r crossplattform and u can make mods with that