Wikipedia defines a database index as follows:
A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space.
Let's use an example to understand this. A library has a catalog of all the books at its disposal. If you want to look for a particular book, you will quickly search through the catalog instead of searching through every isle or shelf for that book. The catalog acts as an index of all the books.
In the computing world an index is something similar. It saves you the trouble of having to search through every record in the database. Instead, you speed up your query by searching a subset of data that is highly optimized for quick reference. This set of data is called an index and it is separate from the original data stored in the database.
To give you a better picture of this, the following table relates a Library to a Database.
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Library |
Database |
|---|---|
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Library is a... |