Reusing Jars
Its perfectly acceptable to reuse your canning jars. Your cost over time for canning gets lower and lower everytime you reuse a jar which is pretty cool. Even if you don't need to save the bucks the reality is throwing away a prefectly good, albeit used, jar is lame. All of us, myself included, need to get more used to the idea of getting maximum milage out of the things we buy.
The same cautions apply with reused jars. Make sure that they are clean and free from cracks, nicks and chips. Reuse the tightening rings as well.
You cannot reuse the canning lids or rubber gaskets. You'll need to buy these new each time.
*note*
Exercise some caution in reusing store bought jams and jellies that come in jars that "look" like canning jars. First off commerical jars are cheap and chances are high that they will break when you try to can with them. This is particularly likely when doing pressure canning. Its probably never a good idea to use commercial jelly/jam jars for pressure cooking.
The next problem is that commercial canning jars come in irregular sizes. So if you think you have ajar that you can reuse double check to ensure that your lids will fit snuggly and completely. Also, make sure that the tightening ring can screw on completely without coming loose.
If you have even a little doubt about reusing your own canning jars or using commercial ones don't use them.