DAG-PB is a codec that implements a very small subset of the IPLD Data Model in a particular set Protobuf messages.
DAG-PB can be parsed by Protobuf libraries, and can be emitted by Protobuf libraries (with some caveats -- DAG-PB is actually stricter than Protobuf in some details).
DAG-PB implements only a very small fraction of the IPLD Data Model. You should not expect to be able to take arbitrary IPLD documents and emit them as DAG-PB, nor to be able to take documents from other codecs (e.g. DAG-CBOR) and transliterate them into DAG-PB -- DAG-PB isn't that flexible.
DAG-PB is mostly seen in IPFS, where it is a backbone of how Unixfsv1 data is serialized.
See the DAG-PB specifications for details.