Foreign and domestic dividends are often taxed differently.

Investors who hold shares of domestic companies typically receive the full amount whenever dividends are paid. At the end of the year, investors will report any dividend earnings to their national tax authority (say, the IRS) and pay taxes accordingly.

By contrast, investors who hold shares of foreign companies are often subject to foreign dividend withholding tax. In other words, whenever a company pays a dividend to a foreign investor, the foreign tax authority will automatically withhold a portion of that payment as tax. As some markets withhold up to 35%, foreign dividend withholding tax can significantly lower the return on international investment.

For reference, a list of popular foreign securities can be found here.