A Document Shepherd makes sure an Internet-Draft (I-D) continues to progress appropriately through the IETF processes, keeping in mind the overall IETF goal of producing technically excellent documents.
A document won't shepherd itself. Chairs are often occupied with other aspects of Working Group (WG) operations. An Area Director has to worry about many other documents. Authors sometimes vanish. WG Chairs therefore designate an individual as the shepherd for working group I-Ds.
A Document Shepherd takes responsibility for pushing a WG document along. Firstly, to the point where the WG has consensus, and then, if that's acheived, through the Internet Engineering Steering Group (IESG) review, and finally through the RFC Editor (and IANA) processing.
It is considered generally preferable for a Shepherd to not be a Chair of the working group, or from the same organization/company as the Chair.
More detail about the document shepherding process can be found in:
One of the key things a document shepherd does is to write up the history of a document's path through the Working Group (WG) process. The purpose of the shepherd writeup is to make sure that the document has been properly reviewed and to provide information to the IESG for use in IESG Evaluation. The IESG has provided a writeup template for individual submissions and a writeup template for working group submissions as a guides.
The document shepherd writeup is not subject to working group consensus. Quite the opposite, it's the place for the shepherd to communicate with the Area Directors, and SHOULD include information that the shepherd thinks is important, even or especially if the WG isn't in agreement.
The writeup should be explicit about which document the writeup is about and its target status (e.g. Informational, Proposed Standard). Working Group charters or milestones are sometimes unclear about the intended status of a document. And, the WG may decide to change the intended status based on consensus (should be discussed with your AD). Therefore, to avoid problems with this and also make it clear in the WG process what is happening it's strongly recommended that the WG chairs do the following.