In case of a flight delay of more than 3 hours after the scheduled arrival, you are maybe entitled to compensation. The amount of this compensation depends on several factors, which are mentioned elsewere on this page.
If the delay is more than 5 hours, you may decide not to travel (or continue to travel).  In that case you are entitled to a refund of the ticket price and also to a return flight to your initial departure point.
The exception to the rule is force majeure (extraordinary circumstances). Your rights expire when the airline can prove that there is force majeure, such as:

• strike of air traffic control or airport employees
• a medical emergency landing
• terrorism
• bad wheather conditions (like snowstorm)
• natural disaster

According to the European Court of Justice, technical defects are no force majeure, allthough many airlines do appeal to this. Strikes by their own staff are also not covered by the term force majeure.  In these situations you are entitled to compensation.

Eligibiliti to a refund or compensation will only be possible when:
your flight has departed from an EU member state,  or to an EU member state with an airline based in an EU member state.

For flights operated by an airline that is not based in an EU member state,  the rule applies that that you will only be compensated, when you departed from  an EU member state.

You are also entitled to compensation if you fly with a transfer switch within the EU to a destination outside the EU.