xDS is a set of discovery services (hence the name x Discovery Service or xDS) used to convey routing information for a service mesh or load balancing. I'm aware of 2 main users: Envoy (who primarily designed it) and gRPC.
Envoy's blog on universal data plane is probably the best intro into why/how the data and control plane are separated and how the api came to be. The gRPC xds proposal introduces a lot of the concepts more cleanly than the envoy docs.
As a set of APIs defined in protobuf/gRPC, The definitions can be found in the main envoy repo under envoy/api/envoy/service mirrored to the envoy/data-plane-api repo for consumption. cncf/xds appears to be a future home for it? There are also reference control plane servers, ex: go-control-plane.
Listener: Local L4/L7 listener + filters for proxy
Route: HTTP routing table
Cluster: Upstream to forward requests to
Endpoint: Backing hosts for upstreams
ADS: Aggregated Discovery Service
CDS: Cluster Discovery Service
CSDS: Client Status Discovery Service
EDS: Endpoint Discovery Service
LDS: Listener Discovery Service
RDS: Route Discoery Service
RTDS: Runtime Discovery Service
SDS: Secret Discovery Service
SotW: State of the World
SRDS: Scoped Route Discovery Service
VHDS: Virtual Host Discovery Service
UDPA: Universal Data Plan API
DPLB: Data Plane Load Balancer
CPLB: Control Plane Load Balancer
LRS: Load Reporting Service
ORCA: Open Request Cost Aggregation