The registry consists of a few trees and can therefore be considered a «hierarchical database».
The nodes in the tree are keys, the leaves are values. Thus, a key consists of (potentially zero) keys (which are subkeys to that key) and of (potentially zero) values.
The names of the keys are not case sensitive and cannot include the backslash \ nor the NUL \0 character and cannot be empty.
The names of the values can contain the backslash or can be empty.
A key can have an (unnamed) default value. With reg.exe, these are referenced with the /ve option.
A key has a 64-bit FILETIME object that contains the LastWrite value. The UserAssist keys maintain timestamps with their data.