MySQL official definition of indexes: Index (Index) to help MySQL efficiently get the data structure of the data. Therefore, the index is essentially a data structure. An index is to improve query efficiency, analogy dictionaries, directories and other books in this form.
It can be simply understood as "Quick Find sorted data structure." In addition to the data, the database system also maintains a data structure to meet specific search algorithm, the data points to the data structure in some way, so that you can implement advanced search algorithms on that data structure, this data structure is indexed.
In general, the index itself is also great, it can not all be stored in memory, so the index is often in the form of index files stored on disk.
The index is usually referred to, if not specified, it is the B-tree index. Wherein the aggregation index, secondary index, covering index, prefix index, the default is a unique index B-Tree.
You can view the table of the index case by show index from tablename.