Concordances are books that allow a user to find every occurrence of a word in the Bible. Concordances are based on either the English translation that is being used, e.g. NRSV, or alphabetical arrangement of the original Biblical language, e.g. according to the Greek alphabet. The concordance will present the word and that will be followed with a list of passages in which that word occurs.
The Strong's concordances are based on the work of one individual, named James Strong, not so ironically. Strong set about arranging the Hebrew words used for the King James Version of the Bible alphabetically. He then numbered the root word. He then did the same for the Greek of the New Testament. The concordance was then arranged by the numbers, which correspond to a root word found in the Bible. Newer editions of the Strong's concordance are not based on the KJV, but on newer translations.