What is B+ tree?

The fundamental data structure used in all major database systems storing the data. B+ tree is a variation of B tree, both have a very large fanout to store indexes to the data page, which greatly reduces the number of I/O operations to find the desired data. Unlike B tree which stores data on both leaf and non-leaf node. B+ tree only stores data on the leaf node and index data on the non-leaf node, which gives B+ tree a even large fanout on the intermediate non-leaf index node.