Where can I find out about my mother-in-law's ancesters on line?

Question:

Where can I find out about my mother-in-law's ancesters on line?

She was born in PA, maiden name Chapman, married a Paul Mason from near Williamsport PA

Best Answer - Chosen by Asker: YOu don't indicate if she is alive or dead.
If she (or her husband) are dead, I'd expect to find them in the social security death index (assuming they died in the last 50 yrs). This provides dates and place of death. Death certificates should provide the names of both parents, and their place of birth.
There is also a huge volunteer network of county based web pages, called USgenweb. Each of these sites have a varied level of genealogical records based on that specific locality. This could be marriage records, obituaries, family biographies, census, etc.
There are tons of records online, however they cannot cover all available records. You can get a lot done, assuming the ones you need are there.. some things won't be found. Every thing depends on the person, the date, the place.
I also suggest that there are beginner's tutorials, at www.cyndislist.com. These are great for explaining what records you might use, and how to track them down.