Tax Havens are countries etc that allow business or people to avoid or significantly reduce the amount of tax they might pay.
Places like China (Hong Kong inclusive) has a corporate tax rate of 17% which is more than half that in the United States which would be an attractive tax haven for companies.
The Organisation for Economic Cooperation and Development (OECD) is an organisation that wants to ruin their party and tries to get all countries to implement fair taxes across the world. They claim, "In 2000, we identified 47 potentially harmful preferential tax regimes in OECD countries. Of those regimes, 19 regimes have been abolished, 14 have been amended to remove their potentially harmful features, 13 were found not to be harmful and only one has been found to be harmful."
Zyra has a tax haven list with countries and some brief information about them.