Given a connected and undirected graph, a spanning tree of that graph is a subgraph that is a tree and connects all the vertices together. A single graph can have many different spanning trees. A minimum spanning tree (MST) or minimum weight spanning tree for a weighted, connected, undirected graph is a spanning tree with a weight less than or equal to the weight of every other spanning tree. The weight of a spanning tree is the sum of weights given to each edge of the spanning tree.

How many edges does a minimum spanning tree has? 
A minimum spanning tree has (V – 1) edges where V is the number of vertices in the given graph. 
What are the applications of the Minimum Spanning Tree? 
See this for applications of MST.

Kruskal’s Algorithm for Minimum Spanning Tree  : https://www.geeksforgeeks.org/kruskals-minimum-spanning-tree-algorithm-greedy-algo-2/