A short answer is that a self-organizing work group is a grassroots group of people who come together of their own accord to get work accomplished in an organization. I’m a longer-answer person myself, so here’s a definition I’m working on. A self-organizing work group (SOWG) is a collection of people stepping outside of their formal organizational structure to collectively take responsibility for a complete work process or project. Within the group, roles and jobs are defined and redefined as needed by group members so they can work in more interchangeable ways and so that the group can function in more flexible, organic ways (adapted from Morgan, 1997). These groups appear to come into being without much planning and to emerge from local interactions among people pursuing their individual agendas (adapted from Arrow, McGrath, & Berdahl, 2000). In these groups, internal and emergent forces prevail (McClure, 2005). My own experience in these groups and formal research of them has taught me that:

Human imagination and experience play key roles in these groups.
People who become self-organizing group members first imagine and experience a connection between their own fundamental need/belief/tendency and the fundamental need/belief/question of the people who matter most to them in their organization. Eventually, the group itself imagines and experiences a connection between its own fundamental need/belief/tendency and the fundamental need/belief/question in an even larger system/organization.

They spread organically.
The pattern of growth of these groups is organic in that their ideas spread in their larger systems/organizations by going anywhere that others open to a new way of working and thinking recognize their usefulness in addressing related needs in themselves and those who matter most to them. That is, their ideas follow a path of most acceptance—similar to the “path of least resistance” idea demonstrated by river water flowing the easiest way to the sea.

Their impacts and their limitations are important parts of the whole.
These groups deeply impact the individuals in them and can deeply impact people near them and close to group members who are open to working and thinking in a new way as well. However, for the ideas they are demonstrating to spread beyond their local influence, those near the group and close to group members must self-organize themselves, demonstrating the ideas to additional others. It appears that only people in direct contact with these groups can sense and see what matters most to them—and what truly makes them effective—because what matters most to them and what makes them effective cannot simply be talked about (and often isn’t). It must be demonstrated. This limitation is an important part of what ultimately makes these groups so powerful.

These groups aren’t necessarily easy to see.
So why don’t we know more about groups like these? I think its because they can be difficult to see, even for people who are part of them. They don’t look and act like the formal “teams,” “departments,” “groups,” and “divisions” that we’re used to thinking about (with people in charge, formal goals and structure, etc.). They emerge partially into view where you can see that the people in them are serving both their own needs and the needs of those who matter most to them (“Hey, those people are smart and they care—they are savvy but not selfish. I like that.”). They emerge more fully into your view at the point at which they demonstrate value for yourself and those who matter most to you (“What they are doing will benefit me and the people who matter most to me.”). They emerge even more fully when you self-organize yourself.