A lexicon is similar to a dictionary but offers much more than definitions to aid a user in comprehending the "nuance, context, and possible meanings" of a word. Also, some lexicons provide examples of a word in context from actual primary texts.
Tyndale Tech out of Tyndale House (Cambridge, UK) highlights the usefulness of lexicons and points to some online versions that "make real lexicons easier to use." Check it out.
The word "lexicon" derives from the Greek λεξικόν (lexicon), neuter of λεξικός (lexikos) meaning "of or for words",[1] and represents the inventory of words (lexemes) in a language.
You will also come across "analytical" lexicons, which (according Davidson's Analytical Hebrew and Chaldee lexicon) consist of "an alphabetical arrangement of every occurring inflection of every word contained in the Greek or Hebrew Scriptures, together with a grammatical analysis of each word, a lexicographical illustration of the meanings, a complete series of paradigms, with grammatical remarks and explanations."
A Greek-English Lexicon by 
A Greek-English Lexicon of the New Testament and Other Early Christian Literature by 
A Greek-English Lexicon of the Septuagint by 
Vocabulary of the Greek New Testament by 
A Patristic Greek Lexicon by 
The Hebrew and Aramaic Lexicon of the Old Testament by 
The Brown-Driver-Briggs Hebrew and English Lexicon by 
Dictionary of Classical Hebrew by 
Hebrew in Its West Semitic Setting by 
An Akkadian Lexicon Companion to Biblical Hebrew by 
A Dictionary of the Targumim, the Talmud Babli by