There are two general philosophies translators use when they do their work: formal or complete equivalence and dynamic equivalence. Formal equivalence translations try to give as literal a translation of the original text as possible. Translators using this philosophy try to stick close to the originals, even preserving much of the original word order.
Literal translations are an excellent resource for serious Bible study. Sometimes the meaning of a verse depends on subtle cues in the text; these cues are only preserved by literal translations.
The disadvantage of literal translations is that they are harder to read because more Hebrew and Greek style intrudes into the English text. Compare the following renderings of Leviticus 18:6-10 from the New American Standard Bible (NAS—a literal translation) and the New International Version (NIV—a dynamic translation):
The NAS reads: "None of you shall approach any blood relative of his to uncover nakedness. . . . You shall not uncover the nakedness of your father’s wife; it is your father’s nakedness. The nakedness of your sister, either your father’s daughter or your mother’s daughter, whether born at home or born outside, their nakedness you shall not uncover. The nakedness of your son’s daughter or your daughter’s daughter, their nakedness you shall not uncover; for their nakedness is yours."
The NIV reads: "No one is to approach any close relative to have sexual relations. . . . Do not have sexual relations with your father’s wife; that would dishonor your father. Do not have sexual relations with your sister, either your father’s daughter or your mother’s daughter, whether she was born in the same home or elsewhere. Do not have sexual relations with your son’s daughter or your daughter’s daughter; that would dishonor you."
Because literal translations can be difficult to read, many have produced more readable Bibles using the dynamic equivalence philosophy. According to this view, it does not matter whether the grammar and word order of the original is preserved in English so long as the meaning of the text is preserved. This frees up the translator to use better English style and word choice, producing more readable translations. In the above example, the dynamic equivalence translators were free to use the more readable expression "have sexual relations with" instead of being forced to reproduce the Hebrew idiom "uncover the nakedness of."
The disadvantage of dynamic translation is that there is a price to pay for readability. Dynamic translations lose precision because they omit subtle cues to the meaning of a passage that only literal translations preserve. They also run a greater risk of reading the translators’ doctrinal views into the text because of the greater liberty in how to render it.
For example, dynamic Protestant translations, such as the NIV, tend to translate the Greek word ergon and its derivatives as "work" when it reinforces Protestant doctrine but as something else (such as "deeds" or "doing") when it would serve Catholic doctrine.
The NIV renders Romans 4:2 "If, in fact, Abraham was justified by works (ergon), he had something to boast about—but not before God." This passage is used to support the Protestant doctrine of salvation by faith alone. But the NIV translates the erg- derivatives in Romans 2:6-7 differently: "God ‘will give to each person according to what he has done (erga).’ To those who by persistence in doing (ergou) good seek glory, honor and immortality, he will give eternal life."
If the erg- derivatives were translated consistently as "work" then it would be clear that the passage says God will judge "every person according to his works" and will give eternal life to those who seek immortality "by persistence in working good"—statements that support the Catholic view of salvation.
Even when there is no doctrinal agenda involved, it is difficult to do word studies in dynamic translations because of inconsistency in how words are rendered. Beyond this, the intent of the sacred author can be obscured.
more at http://www.catholic.com/library/Bible_Translations_Guide.asp