CrateNum
DefId
A DefId identifies a particular definition, by combining a crate index and a def index.
DefIndex
A DefIndex is an index into the hir-map for a crate, identifying a particular definition. It should really be considered an interned shorthand for a particular DefPath.
DefPathHash
A DefPathHash is a fixed-size representation of a DefPath that is stable across crate and compilation session boundaries. It consists of two separate 64-bit hashes. The first uniquely identifies the crate this DefPathHash originates from (see StableCrateId), and the second uniquely identifies the corresponding DefPath within that crate. Together they form a unique identifier within an entire crate graph.
LocalDefId
A LocalDefId is equivalent to a DefId with krate == LOCAL_CRATE. Since we encode this information in the type, we can ensure at compile time that no DefIds from upstream crates get thrown into the mix. There are quite a few cases where we know that only DefIds from the local crate are expected; a DefId from a different crate would signify a bug somewhere. This is when LocalDefId comes in handy.
LocalModDefId
ModDefId
StableCrateId
A StableCrateId is a 64-bit hash of a crate name, together with all -Cmetadata arguments, and some other data. It is to CrateNum what DefPathHash is to DefId. It is stable across compilation sessions.

Constants

CRATE_DEF_ID
CRATE_DEF_INDEX
The crate root is always assigned index 0 by the AST Map code, thanks to NodeCollector::new.
LOCAL_CRATE
Item definitions in the currently-compiled crate would have the CrateNum LOCAL_CRATE in their DefId.

Module rustc_span::def_id