After the grapes have been harvested it is often a mad dash to get them crushed and destemmed. The objective of crushing is not necessarily to squeeze all of the juice out of the grape, but to split the external skin and allow the juice to start its run, giving the sugar from the juice its first chance to mingle with the natural yeast found on the grape's skin. It's the combination of yeast and sugar that produces the wine's alcohol, via the yeast converting the sugar to alcohol and carbon dioxide. The "crush" happens in one of two ways - again by mechanical means with a heavy spiraled steel roller or the more traditional approach that you see in all of the fun Italian wine-themed movies - the famous grape "stomp."
At this point the grape stems are separated from the juice, or "must" as it is referred to at this phase in the game. This is also the juncture where red wine grapes and white wine grapes take different paths. If a wine is destined to be a red wine then the grape skins (not the juice, which is virtually clear) that provide the color characteristics and the tannin contributions for a red wine. However, if the goal is a white wine, then the grape skins are removed along with the stems at this phase of the process and the grapes are pressed prior to fermentation.