1. Simple loop: a loop in the vertex sequence of a graph, in which all vertices except the first vertex and the last vertex will not appear repeatedly.

2. Main cycle: If any connected branch is added to the tree, it can form a cycle with other tree branches, so it only contains one connected cycle.

Second, the characteristics are different.

1. Simple loop: A path or loop does not contain the same edge repeatedly.

2. Main loop: A path in the graph contains each edge exactly once.

Third, the traversal method is different.

1. Simple loop: Start from a node, and then find a loop path from this point back to this point. This method cannot guarantee to traverse every edge. If the edge of a point has not been traversed, let this point be the starting point and this edge be the starting point edge, which is connected with the current ring. Do this until all the edges have been traversed.

2. Main loop: each node has zero or more child nodes; Nodes without parent nodes are called root nodes; Each non-root node has one and only one parent node; Except the root node, each child node can be divided into multiple disjoint subtrees. In a connected graph, the branches that make up a tree are called branches, and other branches are called connected branches.

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