In short, an Adventure Game is a game that is somewhat like the game Colossal Cave Adventure. That was the first Adventure Game and where the genre gets its name. Much like a Rogue-like is a game that is somewhat like the game Rogue. But, also like Rogue-likes, that's not the end of the story of where the genre has ended up.

Another great early pioneer of adventure games was Zork, and and in 1980, Sierra Online made the first Graphical Adventure Game called Mystery House. Sierra went on to become the most successful Adventure Game company ever with games such as the King's Quest series, Space Quest series, Police Quest series, Phantasmagoria, Quest for Glory series, Gabriel Knight series, and many others. At one point, Sierra Online was the biggest game company in the world until their tragic downfall in the late 90's.

Sierra's biggest competition was LucasArts which made classics such as Maniac Mansion, Day of the Tentacle, the Monkey Island series, Sam & Max, Full Throttle, and Grim Fandango. Although they made fewer games than Sierra, it is often a point of debate among Adventure Game fans which of these two companies made the best Adventure Games. (Hint: They both made great games)

There are, of course, many other companies who have made and continue to make Adventure Games. Contemporary Adventure game Companies include Wadjet Eye Games, Grundislav Games, and of course Goloso Games and Stand Off Software!

So, what is an Adventure Game? Basically, it is a game continuing and building upon the style of games like King's Quest, Space Quest, Monkey Island, Myst, Broken Sword, etc.

If you are  unsure, the best thing you can do is play the top games from previous years of this jam. They are all free, relatively short, quite good, and they are all Adventure Games.

Here is a definition of Adventure game from Wikipedia

Essential elements of the genre include storytelling, exploration, and puzzle-solving. Marek Bronstring, former head of content at Sega has characterised adventure games as puzzles embedded in a narrative framework; such games may involve narrative content that a player unlocks piece by piece over time. While the puzzles that players encounter through the story can be arbitrary, those that do not pull the player out of the narrative are considered examples of good design.

Combat and action challenges are limited or absent in adventure games; this distinguishes them from action games. In the book Andrew Rollings and Ernest Adams on Game Design, the authors state that "this [reduced emphasis on combat] doesn't mean that there is no conflict in adventure games ... only that combat is not the primary activity." Some adventure games will include a minigame from another video-game genre, which adventure-game purists do not always appreciate.