Hi Teresa, take a look at Rachael's project for starters. She refinished 100 year old hardwood flooring: http://www.hometalk.com/492999/refinished-100-year-old-hardwood-flooring Is this something you're looking to do yourself?
Teresa...I sanded my first hardwood Floor back In 1983...and since then have done enough to fill dozens of homes.
Depending on how worn the finish is you will need to do a full sand or just a screen and top coat. If the finish is just dull in some areas the screen and top coat is the quickest and easiest. If the finish however is worn to wood is some high traffic areas or you have some leveling issues then a full sand is required.
The sanding part is the toughest and can lead to some damage if not done properly. There are a number of machines that can accomplish this. Some are way more aggressive than others and can speed the job but these also have the most potential for damage.
These range from large 240 V belt sanders, 120 V drum sanders to multi-head orbitals and pad sanders to 16 and 17" floor buff machines.
As a novice you would be better off starting with a pad (rectangular orbital) machine These are the most forgiving but can be a bit slower in removing the old finish. I n most cases an edge tool is needed as well. The trick to a great finish is to work you way completely through each finer grit with a thorough clean up between stages.
Personally I a fan of oil based Poly in satin , but there are a number of other finish options...as I explain in this article I wrote
if you have more specific question fire away.
What do you mean by screen? I have hardwood that has a few scratches in the finish but it is not anything major. What do you recommend?
Screenng is often done as a step just before the last coat of finsh s applied. This is done with a Sanding "screen" and an orbital floor buff type machine. This very light "sanding" abrades the finish for the next coat to bond a bit better.