Fat tree: It is a customized version of tree network. In this network bandwidth of edge (or connecting wire between nodes) increases in the direction of root. It's a more realistic imitation of normal tree where branches get thicker in the direction of root. It's more popular in comparison to structure since practically the more traffic takes place towards the root as compared to leaves therefore if bandwidth does not change the root would be a bottleneck causing more delay. In a tree this problem is avoided due to higher bandwidth.

Figure: Fat tree