Trade
Before December 31st: United Kingdom in the single market and the customs union of the European Union. No trade barriers or tariffs.
January 1 and later: There are no tariffs or quantitative limits on imports or exports, but there are a lot of new border procedures because the UK is outside the EU single market and its customs union.
Customs and border control
Before December 31: There are no customs formalities or minimum border controls for products crossing in any direction.
January 1 and later: New costly customs declarations, security, value added tax and others. New controls on food and animal imports into the EU and some increased controls on goods heading from the British mainland to Northern Ireland.
Travelers
Before December 31: Full access to EU and UK citizens for holidays and business travelers (and their pets) in both ways, with automatic health insurance and quick lane access to airports. Citizens have the right to settle and work in the EU and the UK