Simple graph: acyclic graph without parallel edges; (The ring here is for yourself; )
Parallel edges: refers only to directly adjacent points: A-B; A-B； ... if multiple parallel edges can be drawn between AB; There are also self-rings (multiple self-rings, that is, parallel rings; Or connect yourself)
When you say that AB and BA are parallel sides (that is, two sides between AB), it is not a ring; If ABCA is a triangle undirected graph, it is a circle, but not a ring, it is a simple graph.
ABCA is a discrete ring, not a ring, but a simple ring in the data structure (that is, only the first vertex and the last vertex of the cycle are the same);
ABCADC in undirected graph is not a parallel edge, but also has a circle; (i.e. ABCDA); But in the discrete case, there is no ring, so it is a simple graph (I understand it as a quadrilateral)