This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Indexing is a way of getting to data faster. Just like in a book. If you are trying to find the page that "exporting" is discussed, do you read the entire book or look at the index in the back to find the specific page? Which is faster?
It works the same way for a database. The index helps Oracle find the specific page in memory containing the requested data without looking through the whole table.