A worker cooperative is a business (or other organization) owned and managed by its employees.
There are many different mechanisms for doing this, ranging from simple to complex. A worker coop can be as simple as a business shared by friends who make decisions together. It can be as complicated as a major company employing thousands of people, with each worker owning an equal share of the overall profits and with managers decided by employee election or subject to employee recall.
Worker coops have been around for years, but they're taking on a particular new urgency in the world today. Throughout major regions (especially Europe, Latin America and arguably here in the US) people are losing faith in both the prescriptions of the "Right" (neoliberalism led by multinational corporations and American political and economic leaders) and the "Left" (European-style social democracy manifested in a large welfare state). In Europe, the left is flailng wildly despite popular hostility to big business and militarism.
In Latin America, we've witnessed the birth of a new form of "21st century socialism," called by many the solidarity economy.
We might be at a time when some of the old oppositions between left and right are rconsidered, and the ideas and ideals of worker cooperation, worker ownership and worker self-management exist in exactly that neblous overlap between "conservative" values of personal responsibility and hard work and "liberal" values of community and solidarity. In America we may see this expressed through worker coops and democratic ESOPs, as well as an array of civic economic reforms.
We may yet live to see something new under the sun, a genuine democratization of economic life.