The 2.8 gig is about what I have seen in other Android tablets, which make up the Android system areas of your device and include specific access rules.
The phone's internal memory (not the SD card) is solid-state (flash) memory, AKA NAND. It can be partitioned much like a normal hard drive can be partitioned. The bootloader exists in its own partition. Recovery is another partition; radio, system, cache, etc are all partitions.
Here are the standard partitions on an Android device:
/misc - not sure what this is for.
/boot - bootloader, kernel
/recovery - holds the recovery program (either clockworkmod or RA recovery for a rooted Evo)
/system - operating system goes here: Android, Sense, boot animation, Sprint crapware, busybox, etc
/cache - cached data from OS usage
/data - user applications, data, settings, etc.
The below partitions are not android-specific. They are tied to the hardware of the phone, but the kernel may have code allowing Android to interact with said hardware.
/radio - the phone's radio firmware, controls cellular, data, GPS, bluetooth
Generally the remaining storage is configured as an SD card (your 13.24 GB of Total Space) and is user read and writable. Rooting your Android gives you full access to all partitions.