A commerical website that is adding searchable databases of digitized materials daily to their website. Some of it is free but to get full access, you'll need to PAY A MEMBERSHIP FEE on a monthly or yearly basis. There is a few 14 day free trial available, so try it. I love ANCESTY.COM and use it often to trace my relatives in their US Federal CENSUS databases which are indexed by states, and show you copies of the actual census forms 1790-1930. Lots of errors in censuses but they have great CLUES to where your ancestors lived and other info. depending on the year of the census. They can tell you: approximate age, occupation, marriage status, race, where born, where their parents were born, when immigrated, if they served in the military, if they were literate, etc. A must do for those searching in USA.
Lots of tutorials, free forms and helps on their website including a weekly free informative newsletter. Their maps are excellent also. Many different kinds of records are available and searchable including: vital records, court records, land, probate, books, military, newspapers and more. They have records of other countries here also-check it out. Some local LDS Family History Centers have free access to ANCESTRY. (Most local public libraries have a database called HERITAGE QUEST which will enable you to check the Federal censuses for FREE with only your library card # instead of relying on ANCESTRY.)
A great NEW feature on ancestry.com is FAMILY TREE which enables you to upload your family history information and post it for others to add to and for you to continue working on. The fun thing about this feature is it allows you to link VITAL RECORD certificates or CENSUS forms you discover on Ancestry with your ancestor then you can easily access it. You can also upload photos or GEDCOMS (genealogical data communication) files that can be shared between users of any of the software programs like PAF or Family Treemaker or Legacy. ANCESTRY has enabled me to find some Internet cousins of my husband's who had a ton of info and photos to share. If you're really serious about family history research ANCESTRY is well worth the money to have access to it 24/7. Then you know you are really addicted to GENEALOGY!