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+# Path types
+
+In graph theory, a path in a graph is a finite or infinite sequence of edges which joins a sequence of vertices. Paths are fundamental concepts of graph theory.
+
+Paths can be categorized into 3 types: `walk`, `trail`, and `path`. For more information, see [Wikipedia](https://en.wikipedia.org/wiki/Path_(graph_theory)#Walk,_trail,_path).
+
+The following picture is an example for a brief introduction.
+
+![path](../../images/path1.png)
+
+## walk
+
+A `walk` is a finite or infinite sequence of edges. Both vertices and edges can be repeatedly visited in graph traversal.
+
+In the above picture C, D, and E form a cycle. So, this picture contains infinite paths, such as `A->B->C->D->E`, `A->B->C->D->E->C`, and `A->B->C->D->E->C->D`.
+
+!!! note
+
+    `GO` statements use `walk`.
+
+## trail
+
+A `trail` is a finite sequence of edges. Only vertices can be repeatedly visited in graph traversal. The Seven Bridges of Königsberg is a typical `trail`.
+
+In the above picture, edges cannot be repeatedly visited. So, this picture contains finite paths. The longest path in this picture consists of 5 edges: `A->B->C->D->E->C`.
+
+!!! note
+
+    `MATCH`, `FIND PATH`, and `GET SUBGRAPH` statements use `trail`.
+
+There are two special cases of trail, `cycle`, and `circuit`. The following picture is an example for a brief introduction.
+
+![trail](../../images/Circuits1.png)
+
+- cycle
+
+   A `cycle` refers to a closed `trail`. Only the terminal vertices can be repeatedly visited. The longest path in this picture consists of 3 edges: `A->B->C->A` or `C->D->E->C`.
+
+- circuit
+
+   A `circuit` refers to a closed `trail`. Edges cannot be repeatedly visited in graph traversal. Apart from the terminal vertices, other vertices can also be repeatedly visited. The longest path in this picture: `A->B->C->D->E->C->A`.
+
+## path
+
+A `path` is a finite sequence of edges. Neither vertices nor edges can be repeatedly visited in graph traversal.
+
+So, the above picture contains finite paths. The longest path in this picture consists of 4 edges: `A->B->C->D->E`.
diff --git a/docs-2.0/1.introduction/2.data-model.md b/docs-2.0/1.introduction/2.data-model.md
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@@ -8,7 +8,7 @@ Nebula Graph data model uses six data structures to store data. They are graph s
 
 - **Graph spaces**: Graph spaces are used to isolate data from different teams or programs. Data stored in different graph spaces are securely isolated. Storage replications, privileges, and partitions can be assigned.
 - **Vertices**: Vertices are used to store entities.
-  - In Nebula Graph, vertices are identified with vertex identifiers (i.e. `VID`). The `VID` must be unique in the same graph space. VID should be int64, or fixed_string(N)。
+  - In Nebula Graph, vertices are identified with vertex identifiers (i.e. `VID`). The `VID` must be unique in the same graph space. VID should be int64, or fixed_string(N).
   - A vertex must have at least one tag or multiple tags.
 - **Edges**: Edges are used to connect vertices. An edge is a connection or behavior between two vertices.
   - There can be multiple edges between two vertices.
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