
There are three main types of concordances.
The English dictionary kind – This type lists all the occurrences of a key word with a snippet of the verse referenced. Cruden's Complete Concordance is an example. It's perfect for those times when you can remember part of verse but have no idea where it is. Look up one of the key words and use the verse snippets to track down the one you need. One note on Cruden's – it is based on the King James, so if you don't have a good idea how the verse is rendered in that translation you may strike out on your search from time to time.
The other types are original language concordances. They differ in how they are indexed.
Arranged by original language word – THE STANDARD is Strong's Exhaustive Concordance. First published in 1890, it assigned a number to every Hebrew and Greek root in the entire Bible. Every single one. You can find Strong's Concordances for KJV, NAS and NIV. Mine has the Biblical text with the numbers as superscripts. In the back, there is a Hebrew and Greek dictionary arranged by number. Other editions of the concordance just have the dictionary part. I've also seen "concise" editions that are just the English language dictionary like Cruden's. If you get a Strong's make sure you're getting the kind you want, otherwise it will probably end up as a door stop.
Arranged by English word – The other original language concordance isn't as popular, but in some ways, it's more intuitive. Young's Analytical Concordance is arranged by the English word. Look up the word servant, for example, and you'll find a dozen or so different words have been translated into one English word. Each one gives an extra insight.
So, why would you want one? After all, they weigh twenty-five pounds each and they are full of stuff that's not even English.
That's just it. While the translators do their level best, they can't tell us everything the original text does. Over 12000 original language words end up as around 6000 English words. A concordance can help you dig down and discover some good stuff.
Let me give you an example. When Noah built his ark, God told him to coat it with pitch. The Hebrew word is kaphar. That's the very same word God uses later to tell us the blood of the sacrifices covers sins. There it's translated atonement. It's also translated as mercy, reconciliation, purged, cleansed, forgiven, pacified, appeased and pardoned. All those ideas are carried in that one word. When you know that, and then read Christ our atonement, it gives a richer, fuller idea of Christ's work on our behalf. That's why I use a concordance (or two).
Do you use a concordance? What have you learned?