"Global" searches of all databases on Ancestry.com will usually pick up many of the best results, but will also typically miss at least a few gems. To be sure you have truly explored everything Ancestry.com has to offer, you need to dig a little deeper and finesse your searches at the level of each individual database that potentially relates to your ancestor. This is why every search for your ancestors on Ancestry.com should include some time spent in the Ancestry Card Catalog.
Similar to the card catalog you've probably used at the library, the Ancestry Card Catalog is a gateway to the databases on Ancestry.com. There are currently more than 30,000 database titles on Ancestry.com, each containing anywhere from a 1500 names (German directory "Adreßbuch der Direktoren und Aufsichtsräte, 1933, 2. Band") to over a billion names ("Public Member Trees"). A search initiated from the Ancestry.com home page or from "Search All Records" on the Search tab dropdown, searches all of those databases at one time. This is usually a good way to start your search, but has its own set of problems. Sometimes a global search returns just too many results to be immediately useful. If you've spent a lot of time on Ancestry.com, you'll also know that global searches sometimes miss results that the exact same search will turn up if conducted within an individual database.
To directly search the databases that a global search might be missing, you first need to know what's available. This is where the Ancestry Card Catalog is your friend. You can access the catalog from My Quick Links in the upper-right corner of your home page (unless you've customized this list and removed that link), or at the bottom of the drop-down menu of the Search Tab. Interestingly, there appear to be two different versions of the Ancestry Card Catalog - the Ancestry Card Catalog and the Ancestry Database Card Catalog - and each operates slightly differently.
The Card Catalog is fully searchable by either title or keyword, but don't just search for an ancestor's surname and call it a day. Searching by title searches only the words in the title of the database, whereas searching by keyword will also search the extended description of that database. This is the easiest way to find a database for which you already know the title - perhaps you've seen it included as a source in an online family tree or published genealogy. You can also use this to search for databases specific to a particular locality (search by the name of country, state, county, parish, town, etc.).
Another way to use the Ancestry Card Catalog is to browse through all available titles while filtering by Collection (record type), Location, Date and/or Language. I usually find it easiest to begin browsing the titles by Locality, and then narrowing by Collection (Record Type) as necessary. To access the Browse feature, select the link to browse a list of databases in the descriptive paragraph just under Ancestry Database Card Catalog or "View all Ancestry Titles" at the top of the Browse Records section on the right-hand side of the page. If you're using the alternate Ancestry Card Catalog, then you can filter the results list by Collection, Location, Dates or Language in the Filter Titles bar on the left-hand side of the screen.
Whether you browsed or searched, you can sort your results list by popularity, database title, date updated, date added, or record count. Look for the little dropdown menu at the top of your results page. If you don't see one, you may have stumbled into the alternate version of the Ancestry Card Catalog - try this version of the Ancestry Card Catalog instead.