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.
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."