Big Database

     The Big Database 2016 contains more than 6.4 millions games from 1560 to 2015 with ChessBase opening classification with more than 100,000 key positions, direct access to players, tournaments, middlegame themes, endgames. The most recent games of the database are from the Middle of September 2015. 
     You need a large reference database if you’re going doing serious study or correspondence play.   Online databases are OK, but they can’t manipulate the data and making your own requires a lot of time and effort. 
     With BigBase allows you to access various keys for openings, endgames, strategic and tactical themes which is probably the best and most helpful feature.  And, playing over a lot of sample games with your favorite opening lines, endings or middlegame themes is one of the best ways to learn.  When you see similar positions in your own games, you will remember them.  Even if it's only a vague recollection, at least you have a starting point. Learning by example is one of the best ways of learning anything.  Seet his article on Observational Learning.
     Note though that BigBase does NOT include annotated games and you are not offered a year’s worth of weekly updates. Nor does it, like MegaBase, offer rating data and pictures for thousands of players. Personally, I do not think the price of MegaBase (around $135) is worth the extra cash outlay. BigBase 2016 is available for download or by mail.