Libertarian is a strong advocate for maximum amount of freedom is all aspects of our lives (namely social and economic issues), advocates limiting government to it's most basic functions that are clearly outlined in the US Constitution, and holds individual liberty, private property rights, free markets, free association between consenting adults/entities, and individual responsibility and accountability for ones actions as the main platform of the party.
Constitution party is similar to the libertarian party except that they have a more traditionalist approach when pertaining to social issues (so a bit less freedom on social issues), if I remember right.
The Green party is big on environmentalism and is akin to more socialist views pertaining to government, like the redistribution of wealth, higher taxes, government taking over key parts of industry, less economic freedom, etc.
The libertarian platform is strict about adherent to the US Constitution and is what the framers like Benjamin Franklin, Thomas Paine, Thomas Jefferson, and so on, intended the government to be. Not his convoluted big government running our lives crap that the Greens, Republicans, and especially Democrats support. I know you didn't want bias; although this isn't just bias, it's also fact that the other parties are far from the framers intentions for government restraint and responsibilities, and a complete slap in the face to what they gave us.