The number of Bible translations has grown significantly and there seems to be no end in sight. This can be rather confusing for most people since they are, more often than not, unaware of the kinds or purposes of translations and/or what they are based on.
Some Bibles like the New International Version are intended to be easy to read and understand while maintaining fidelity to the Greek and Hebrew manuscripts, which is known as Dynamic Equivalence. Others like the New American Standard Bible are intended to accurately reflect the Greek and Hebrew manuscripts while maintaining a high degree of readability, which is known as Formal Equivalence.
The difference between them is two-fold; first, they are different in emphasis or purpose. Translators are constantly choosing from translations that exist on a continuum between most accurate—the original language—and most readable, which would be the simplest vernacular. The former would maintain accuracy but be unreadable to most, and the latter would be readable to most but dangerously inaccurate.
Second, they differ in regards to what manuscripts they use for their translation. While the King James is a formal translation like the NASB, the translators of the KJV use only some of the ancient manuscripts available; whereas, the NIV and the NASB use all of the available manuscript evidence.
Since the NASB uses all of the available manuscript evidence, is a formal translation, and yet very readable, I recommend that everyone have an NASB for their primary translation or at least for their Bible study. For a more thorough understanding of the differences in translations you can listen to my Messages on “Help!! Which Bible Translation is Right?” or click more for an synopsis.microsoft-word-help-which-translation-is-right-abreviated-4-19-07-rwr-edited-ac.pdf