In Community parlance people often refer to the three pillars of the EU Treaty. These are:
the Community dimension, comprising the arrangements set out in the EC, ECSC and Euratom Treaties, i.e. Union citizenship, Community policies, Economic and Monetary Union, etc. (first pillar);
the common foreign and security policy, which comes under Title V of the EU Treaty (second pillar);
police and judicial cooperation in criminal matters, which comes under Title VI of the EU Treaty (third pillar).
The Treaty of Amsterdam has transferred some of the fields covered by the old third pillar to the first pillar (free movement of persons).
See:
Communitisation
Single institutional framework