Strings

• Before you break out the tools, ask yourself what happened between now and the last time the guitar was setup the way you like it. If it was never setup to your liking, you can afford to be more intrepid. Otherwise, you probably have an insight into the cause that brought about the new buzz. Strings are common culprits. Lighter strings don't buzz as loud, but light strings may buzz in the same setup that won't buzz heavy strings at all. Sometimes, even the same weight strings by different manufacturers can be the difference between buzz and no buzz. You can always go back to what you were using, or raise the action a bit.
  
  

Bridge/Saddle

• There are two sets of string guides: the bridge/saddle assembly and the nut. The nut is the grooved cross-member near the headstock. The bridge and saddle are different on different guitars; they're near the pegs on acoustics and near the pickups on electrics. Between the nut and bridge, the strings play a tight-wire act. Generally, the nut is not adjustable and the bridge usually is. If you can't fix the buzz with slightly heavier strings, raising the bridge a fuzz, or raising the action (the height of the strings from the fret-board) will do it. Be careful, though. There's a right way and a wrong way to do it. You don't want to change the position of your harmonics.
  
  

Truss Rod

• Most guitars have a truss rod running the length of the neck. The neck and fingerboard are glued together and a metal truss rod runs along a hollow groove in the neck. On most guitars, access to the truss rod is hidden by a plug or small piece of ornamentation. Tightening the truss rod creates a gentle bow in the neck, towards the strings. Too much bow can cause fret buzz, so backing off the truss tension may fix it. Again, you've gotta learn some fundamentals of guitar adjustment to do this without creating more new problems then you solve.
  
  

Frets

• Your frets or fret wires should lineup in an arched plane. Sometimes they don't. Occasionally, they never did because they weren't installed correctly, while other times they can actually come loose. Sometimes they wear down, then the action is adjusted, rather than re-fretting the finger-board. Where fret buzz is concerned, check to see if you've got one fret that's worked its way loose and needs to be repaired. It may not be a big deal. If you have a lot of wear, it's time for new wires.