树边：是对图进行DFS后得到的树中存在的边。所有绿色边缘都是树边缘，如下图所示。
Back Edge ：它是一条边(u, v) ，使得v是节点 u的祖先，但不是树的DFS 遍历的一部分。从5到4的边是后边。后边的存在表示有向图中的循环。
考虑下面给出的一个无向图，下图的 DFS 是3 1 2 4 6 5 。在下图中，如果将 DFS 应用于该图，将获得使用绿色边连接的树。
后边和树边之间的表格：
| S.N. | Tree Edge | Back Edge |
| 1 | It connects the node to its descendants. | It connects the node to its ancestors. |
| 2 | It is the path traversed during DFS. | It is the path not visited during DFS. |
| 3 | They can form bridges. | They can never form bridges. |
| 4 | If it is disconnected, the number of connected components may increase. | Even if it is disconnected, the number of connected components remains the same. |
| 5 | It never creates a cycle. | It can create a cycle. |
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