A racing sailboat functions best when the crew work (including the skipper) is coordinated and error free. It's a little like a coreographed dance - everyone has a position and a set of moves to perform in order and with correct timing. There are any number of positions on a racing sailboat (or, if it's big enough a "yacht"). Some of the more common ones are:

• Skipper - get's to give orders and wear the cool hats :-)
  
• Helm - usually the skipper, but not always on larger boats or distance races. Steers the boat.
• Main Trimmer - adjusts the trim of the mainsail, tightening or easing the main sheet and other controls affecting the mainsail.
  
• Jib Trimmer - adjusts the trim of the jib (or genoa), tightening or easing the jib sheets (on either side) and other controls affecting the jib.
• Grinder(s) - the muscle of the boat, operate the winches at the direction of the trimmers. On small boats, this function is performed by the trimmers. On bigger boats, these are separate crew who move from winch to winch as needed.
• Pit - raises and lowers sails, operating from a position typically standing in the entry to the cabin.
• Foredeck - All of the work forward of the mast. Hooking up the sails, helping them through tacks and gybes, and especially, gybing the spinnaker on spinnaker boats.

On larger boats, these functions are all typically separate people and there are many others as well. For example, I crewed on a Farr 45 with a mostly professional crew. My job was to tension the running back after each tack. That's it. But a very important job on a boat of that size. And on smaller boats, one person often performs several functions.  We owned and raced a 26 foot Lindenberg for a bunch of years. I was skipper, helmsman and main trimmer. The pit guy also served as a grinder. And so on. The point is, the jobs differ quite a bit from boat to boat and especially as the boats vary in size.

So, as a new crew, take a look around and see what job might suit you. Start learning the job. Talk to your fellow crew. Look on the internet. Learn the theory. And practice.