Their Ten Principles are basically these:
1. Grassroots Democracy
2. Social justice
3. Ecological Wisdom (environmentalism)
6. Community-based economics (technically communism, with a lower-case "c")
9. Ecological & Personal Responsibility (environmentalism)
10. Future Focus & Sustainability (environmentalism)
Well, that about sums it up.
It seems that they're made up of hippies, pacifists, communists (lower-case), and feminists who don't think the Democratic Party is doing a good enough job.
In 2000, Ralph Nader ran on the Green Party ticket, garnering a mildly impressive 2.74% of the popular vote. It missed the 5% required for public funding, but qualified it to be on the ballot in quite a few states. The Green Party was most popular in Alaska, Vermont, Massachusetts, Rhode Island, Minnesota, California, Montana, Hawaii, Maine, and D.C. (not in that order).