The European Union has developed over the past five decades from the six-member European Coal and Steel Community to the current 27-member supranational organization through the adoption and ratification of treaties. Every action taken by the EU is founded on treaties that have been approved voluntarily and democratically by all EU member countries. A treaty is a binding agreement between EU member countries. It sets out EU objectives, rules for EU institutions, how decisions are made and the relationship between the EU and its member countries. Under the treaties, EU institutions can adopt legislation, which the member countries then implement.
This page links to a few good sources for EU treaties. For current, consolidated versions of the treaties, EUR-Lex and Europa are the best sources.