It contains minimal booter files and kernel, and at //BaseSystem.dmg, a disk image with a stripped-down and tweaked copy of OS X. Recovery HD is marked in the partition table with the type Apple_Boot, but is actually in the normal HFS+ format.
And some other details are different, but actually not that much.) And in addition to Macintosh HD and Recovery, there'll probably be Preboot and VM volumes, and starting in Catalina there'll be a separate 'Macintosh HD - Data' volume holding the user-modifiable parts of the main filesystem. (Update: with newer versions of macOS running from an APFS volume, there'll instead be a single APFS 'container' on the disk the volumes under it will be listed separately under a 'synthesized' device, probably /dev/disk2. Let me start at the beginning: your hard drive (/dev/disk0) has two relevant partitions: Macintosh HD (your regular startup volume), and Recovery HD. It's rather complicated, and actually a lot of the complexity is to avoid wasting space I don't think you can 'reclaim' anything without breaking it.
