Sanctions are legal restrictions on economic and financial dealings, imposed to change the behaviour of a government, an organisation or an individual without using force. They range from a freeze on one person’s bank account to a comprehensive embargo on trade with a whole country.
The important practical point is jurisdictional. A sanction imposed by the United States binds US persons, transactions touching the US financial system, and in some programmes non-US parties dealing with designated persons. A sanction imposed by the European Union binds EU persons and conduct within EU territory. The same company can therefore be prohibited from one counterparty and free to deal with another, depending on where it sits and which currency it settles in. Most evasion techniques on this site exist to exploit exactly that seam.