Working Groups are typically created with a charter that describes the specific problem or deliverables (a guideline, a standards specification, etc.) it has been formed to address. These charters and other details about the required work are often, but not always, developed with input from initial Birds of a Feather sessions. WG charters state the scope of work for group, and lay out goals and milestones that show how this work will be completed.
There are typically over 100 active Working Groups at any particular time, and each is part of the IETF Area that is most relevant to its work. While Working Groups are generally expected to be short-lived in nature, some provide ongoing improvements of established protocols. The IETF Datatracker provides a complete list of all active working groups, with links to their charters, discussion email lists, and other information. Upon completing its goals and achieving its objectives, a Working Group is concluded.
Working Groups usually conduct most of their discussions via email lists. They may also use sessions during IETF meetings or organize working group interim meetings between IETF meetings to address issues arise during email list discussions.
If you are interested in contributing to a particular Working Group, the information about participating in the IETF may be helpful.