HIR (previously known as descriptors) provides a high-level object oriented access to Rust code.
The principal difference between HIR and syntax trees is that HIR is bound to a particular crate instance. That is, it has cfg flags and features applied. So, the relation between syntax and HIR is many-to-one.
HIR is the public API of the all of the compiler logic above syntax trees. It is written in "OO" style. Each type is self contained (as in, it knows its parents and full context). It should be "clean code".
hir_* crates are the implementation of the compiler logic.
They are written in "ECS" style, with relatively little abstractions.
Many types are not self-contained, and explicitly use local indexes, arenas, etc.
hir is what insulates the "we don't know how to actually write an incremental compiler"
from the ide with completions, hovers, etc. It is a (soft, internal) boundary:
https://www.tedinski.com/2018/02/06/system-boundaries.html.