Furan is a very simple molecule. It is a 5 member ring with the formula C4H4O. It looks something like this:

      / O \
     /     \
    /       \
H--/         \--H
   \\       //
    \\     //
   / ------- \
  H           H

This molecule isn't particularly significant. Its main claim to fame is that its the basis for the naming of furanose carbohydrates, which are widely used in biochemistry. It can also be called furfuran.