The European Court of Justice (ECJ) is the Court of Justice of the European Communities, i.e. the court of the European Union (EU). It is based in Luxembourg (unlike most of the rest of the European Union govenance which is based in Brussels and Strasbourg).
The ECJ is frequently confused with the European Court of Human Rights or ECHR (which is in Strasbourg). However, while the ECJ is part of the EU, the ECHR is not. The ECJ and the ECHR are unrelated to each other, except that all EU member states must first be members of the European Convention on Human Rights of which the ECHR is the dispute resolution body.
The ECJ is the supreme EU court. However, individuals cannot bring cases there.
Individual employees of the European Commission and related EU bodies used to be able to sue their employer in the ECJ. However, there is now a lower court called the Court of First Instance which deals with these cases.
The ECJ has several functions. The most commonly heard cases are:
1. Claims by the European Commission that a member state has not implemented an EU Directive or other legal requirement.
2. Claims by member states that the European Commission has exceeded its authority.
3. References from national courts in the EU member states asking the ECJ what a particular piece of EC law means. The EU has many languages, competing political interests etc. and so local courts often have difficulty deciding what a particular piece of legislation means in any given context. The ECJ will give its opinion, which may or may not clarify the point and return the case to the national court to be diposed of. The ECJ is only permitted to aid in interpretation of the EC law, not decide the facts of the case itself.