From Statistics Explained
Extra-EU refers to transactions with all countries outside of the EU: the rest of the world except for the European Union (EU) as it is now, consisting of 27 Member States. The term is used in the context of external trade, balance of payments, foreign direct investment, migration, transport, tourism and similar statistical areas where goods, capital or people moving in and out of the EU are being measured and where the EU as a whole is considered in relationship to the rest of the world. Extra-EU transactions of the EU as a whole are the sum of the extra-EU transactions of the 27 Member States.
Intra-EU, on the other hand, refers to all transactions occurring within the EU. The term can have a different coverage, depending on the perspective taken: the EU as a whole, a Member State, a region or a city, a port or an airport, ...
The exports of a particular Member State, for instance, can be split into two parts, on the one hand to the countries outside the EU, the rest of the world, 'extra-EU', and on the other to the 26 other Member States, 'intra-EU'. Departures from an airport can be 'extra-EU' or 'intra-EU', in the latter case they also include domestic flights, within the Member State.
To complicate matters, the EU has not always had the same composition, due to consecutive enlargements. For this reason, both terms are sometimes further qualified as 'extra-EU-15', 'intra-EU-25' etc.; 'extra-EU-15', for instance, refers to the rest of the world apart from the 15 Member States that in the past made up EU-15.