Event weapons are weapons which is only obtainable by completing a certain tier in an event.
Most event weapons has the most unique characteristics and abilities compared to the weapons purchased in shops. The Hunter Rifle, the first event-exclusive weapon added to the game, is able to instantly kill light-armored players on the body and has a very fast reload. Another example would be the Automatic Shotgun which is able to rapidly fire seven shotgun rounds per clip.
They are usually obtained from the last tier, but there are also a few exceptions such as the Dual Rookie Machine Guns since it is obtained from the last tier of the third week of the Summer Camp event, which isn't the final tier of the whole event.
There are currently 29 event-exclusive weapons in the game. Note that weapon skins don't count as event weapons, while there are only two exceptions. The Plasma Shotgun came from the Ghostbusters 30th event, but can also be purchased in the store for 525 gold. The Stake Launcher came from the Monster Mash event, but is originally purchased from the store in the 2013 halloween update.