At the risk of being unfair by oversimplifing, the NIV and NIrV are "translations", the NASB is a "transliteration."
The NIV and NIrV try to, in English, convey the same meaning as the original texts even if that means that the exact wording is lost in English..
The NIV is written in something like Grade 9 English, the NIrV is written in something like Grade 4 English. The NIrV chooses to use "smaller" words to make it more understandable. Sometimes this changes the meaning a little further away from the original Hebrew or Greek, but it doesn't seem to go too far from the original.
The NASB much more often trys to give a word for word translation even if that means that the "meaning" is lost in English. It compromises on this goal, but it is better than nothing. The "Updated NASB" compromises slightly further for readability (see notes that come with the Translation)
For example. If in the Hebrew, it is clear by the word endings that "he" refers to a specific person but in English it is ambigous, the NIrV and to a lesser degree, the NIV will, rather than saying the pronoun HE will use the actual person's name again. Similarily, if the text refers to "Simon" and it is absoultely clear that it is referring to "Peter", the NIV will use "Peter" rather than sometimes using "Simon" and sometimes using "Peter."
Personally, I feel that having both the NIV (or NIrV) and the NASB or Updated NASB together give the best combination for study. An even better alternative, if one can read Greek or Hebrew, is to combine the NIV and Greek or Hebrew instead of the NASB. Personally I don't know Hebrew so I use NIV/NASB for OT and NIV/Greek/NASB for the NT.
Obviously in Grade 4, it is highly unlikely that any can read Geek and it is still fairly unlikely that we have any who can read Hebrew - (any Children have Jewish parents?)
We will refer to the Hebrew where appropriate, but will primarily use the NIV(or NIrV) and NASB in class.
Version secifics: specifics:
"YHWH" (often pronounced incorrecty Yaw-Way):
NASB: " LORD" (Almost the same as NIV except all 4 capitals are the same size in NASB, the NIV/NIrV uses "small capitals" for the ORD)
"Adonai" is written "Lord" by all 3.
NIV/NIrV "Sovereign Lord"
NASB "Lord God"