Continuing with our extract from the directory list ...
bdb /* external */ heap innobase /* external */ myisam myisammrg ndb
Let's go through the idea of storage engines once more, this time with a list of all the storage engines, both the ones that we produce, and the ones that others produce. We've already mentioned the internal ones so now we'll remark on the directories of the two common external storage engines BDB and innobase.
The BDB, or Berkeley Database, handler, is strictly the product of Sleepycat software. Sleepycat has a web page at sleepycat.com, which contains, among other things, documentation for their product. So you can download Sleepycat's own documentation of the source code in the BDB directory.
As for the innobase handler, which many of you probably use, you'll be happy to know that the comments in the files are reasonably clear (the InnoBase Oy people are pretty strict about comments). There are two chapters about it in this document.