Key pieces of information like names, dates, and places are usually indexed – just enough information to help you find the record. When you discover a record for your ancestor, be sure to look at the actual image. It will likely contain more information than what is available in the index.
If the index listing shows that the image is "Not available" but does provide a film number, you can request the image from FamilySearch using a Photoduplication Request. The process is easy and it's free.
To see the list of collections, go to FamilySearch.org and click on [Search] and then [Records]. Scroll down to find the link to "Browse All Published Collections". New collections are added every few days, so check back often.