Multilevel indexing

How does multilevel indexing improve the efficiency of searching an index file? What is the order p of a B-tree? Describe the structure of B-tree nodes. What is the order p of a B+-tree? Describe the structure of both internal and leaf nodes of a B+-tree. How does a B-tree differ from a B+-tree? Why is a B+-tree usually preferred as an access structure to a data file?