Maps are stored in a folder (containing a .png, .mi, and .map file) in the game Maps folder. This folder is likely C:\Users\[Your User Name]\AppData\LocalLow\Justin Moore\Quick Tower Defence\Maps.

You copy these folders to share with others, and pasting other people's folders here will let you play their maps. After pasting start up the game and it should appear with the other custom maps, after the games included maps.

Anyone can play any map, including those made with the Jungle Expedition adventure so feel free to share and try out different maps. You will need the Jungle Expedition adventure if you want to create maps with its tiles though.