A flight is delayed if it arrives late at its final destination. If a flight arrives 15 minutes or more after the time indicated on the ticket, there is a delay. A claim for financial compensation is only valid if there was at least a 3-hour delay at the final destination.
A flight is cancelled when the scheduled flight does not take place.
In some cases you may be entitled to assistance (overnight hotel stay, return flight, etc.) and a compensation of up to EUR 600 based on Regulation (EC) No 261/2004.
Denying access to the flight for reasons that are not related to the passenger (e.g. overbooking).
A downgrade implies that you have travelled in a lower class than the one mentioned on your ticket, e.g. economy class instead of business class. The airline may have decided this due to an overbooking in business class. You are entitled to compensation. This may be up to 75% of the price you paid for your ticket. Downgrading is not only related to the typical first/business/economy class division, but also to other denominations implying a different in-flight service. (e.g. “economy plus”)