You think that Starfleet is all work and no play?  Well, it's not.  Crewmen would go insane without some recreation.  That's why holodecks were created. Also, they can create almost perfect simulations if one would be needed in certain situations. Working in a similar manner and making life easier for the crew, are the replicators.  Without them, everything would have to be made by hand e.g. food, jewelry, and clothing.  Most of which would never be able to be found in space.
     The holodeck uses 2 systems, the holographic system and matter conversion system.  The holographic system creates the 3-D background.  While the matter conversion system creates "props" using the ship's raw materials.  The things created here exhibit behaviors almost exactly like the real thing.  A person cannot tell the difference between the images and the "props".

     As you can see from the picture, on the left are the "props" which are replicated and on the right are the images that are holographic.  When they come together we have an almost perfect simulation.  Best part is you don't actually move, the holodeck scrolls around you.  Talk about laziness.

     The most basic part of the holodeck is the omnidirectional holo diode (OHD).  Their job is to make the images seen in the holodeck.  But, it also generates forcefields that stimulate the sense of touch of holographic images.  The only problem is this method is limited to what the computer knows.  The other 5 senses are created using regular methods like speakers for sound or incense for smell.
    The replicators work in a similar manner, except that they don't use the holographic system.  The computer has an extremely large database, which at a moment's notice can recreate anything in its database.  It works in almost the same way as the matter conversion technology in the holodeck.  The computer provides the information for the item then it is replicated at a terminal.