There are two main forms: juridical double taxation (same taxpayer, same income, two countries) and economic double taxation (same income taxed at corporate and shareholder level). The first is addressed by tax treaties between countries; the second by participation exemptions, partial-inclusion rules or imputation credits.
Relief methods include the credit method (resident country grants a credit for foreign tax paid, up to the resident country's tax on that income) and the exemption method (resident country exempts foreign income, with or without progression). Switzerland generally uses the exemption-with-progression method for foreign real estate and business income.
Practical implications matter: a Swiss resident with US dividends faces 30% US withholding tax. Under the CH-US treaty this drops to 15% via Form W-8BEN; the residual 15% becomes a credit on the Swiss tax return. Without proper paperwork, the investor effectively suffers both the 30% US tax and the full Swiss tax on the gross dividend.
A French resident owns Italian real estate that produces EUR 20,000 rental income. Italy taxes the rental at source (~30%). Under the FR-IT treaty France exempts this income with progression — meaning the Italian rental does not push French income into a higher bracket.