An event item is a special type of key item, usually used to get an otherwise unobtainable legendary Pokémon.
In the past, they were usually distributed via Nintendo events. This was often limited to certain countries where Nintendo decided to hold events, and even then, they were only held in certain cities where distribution hardware or software could be set up. However, this has changed with the advent of new technology, namely hardware in the form of the Nintendo DS and DS cards, the software of DS games, better access to the internet, and better availability to Wi-Fi access points or hardware to create one.
Event items in Generation IV were at first offered at events in addition to over Wi-Fi, but physical events for items have been phased out in favor of distribution over Wi-Fi. This has two obvious benefits. The first is to players, for as long as one can get Wi-Fi access to the internet, they can receive event items from anywhere. The second is to Nintendo, which saves money from not having to set up events just for items around the world.
Starting Generation IV, all items and Pokémon that are downloaded into a game as a Mystery Gift are also bundled with a Wonder Card. The Generation IV games will only allow a maximum of three Wonder Cards to be stored while the Generation V games will only allow a maximum of twelve Wonder Cards to be stored; to download a new one, an old one must be deleted. It is important to note that while an item may usually require an event to be completed (most commonly receiving the National Pokédex) in order to be delivered, players can still download the Mystery Gift into their game, regardless if they have met those requirements or not. The deliveryman will hold onto a Mystery Gift as long as its Wonder Card is stored, and the player can pick it up anytime once they have completed the conditions for accepting the delivery.