Check the Critical Mass Wikipedia entry.

Critical Mass is often described as an 'unorganised coincidence'. People meet and travel through the streets together.

The purpose of Critical Mass is not formalised beyond the action of meeting and carrying out the event, creating a public space where automobiles are displaced to make room for alternatives. The one agreed upon slogan is We Are Traffic.

All participants, being equal in leadership, are thought to have equivalent claim to their own intentions and the purpose of the ride.

Critical Mass is undeniably linked to the environmental movement, which cites private automobile use as catastrophic to our global and local environment, in physical and social terms.

Generally, the purpose of the event as indicated by the actions of the riders are meant to oppose the domination of the automobile over our urban culture, or to create something different.

However, these things are often interpreted very differently and some riders may even disagree - for example, one might not ride at all for environmental purposes, but because of social justice theories.

Many do not ride to oppose anything; they ride to celebrate in the streets and be social.