The European Parliament decides practically nothing in the modern structure of the political power of Europe.
The European Parliament is one of the to EU law-making bodies. Since 1979, it is directly elected by EU voters every 5 years. The last elections were in May 2014.
Parliament has constitutional-type and ratification powers. It passes EU laws, together with the Council of the EU, based on European Commission proposals. It has budgetary powers, scrutiny over the executive, it hears citizens’ petitions, and appoints the European Ombudsman. A detailed overview of the European Parliament’s role in the European decision-making process can be consulted here.