The G20 is going to be attended by the heads of state and financial ministers of the 20 most powerful countries in the world, and people who run organizations such as the IMF and World Bank. They're looking to ensure their survival over their own faults for the global economic crisis, which will certainly include making decisions for other poorer nations so that they may be used as a step ladder for their own success. There's going to be various protest groups protesting for different things: pro-queer (they feel that gay rights aren't equal and represented in the G20), anti-racist, anti-capitalist, anti-authoritarian, anti-globalization, etc.
The G20 is essentially reinforcing an undisrupted dominant worldview for these 20 countries that only cements the heights of corporate globalization while leaving all the poorer countries behind. Protesters are trying to get to the root causes of the problems of unemployment, differential advantages, sovereign collapse, starvation, poverty etc, and the problems are so big that people are so frustrated that they would even put themselves in jeopardy to protest.
The good thing about this protest is that anarchist groups are getting far more exposure at the moment because they can appeal to the common dislike for the G20 from students, unions, peace groups, marxists and whoever else, and they're at the forefront right now organizing protests and handing out booklets, spreading information. I don't think the protests will achieve much but I think they're going to be pretty big because they can appeal to anyone, those who want to live their "antisocial insurrectionist fantasies" as dietcoke said, and those that just want to go and shout some things in a peaceful protest like most people will.