What is the difference between modern and Biblical Hebrew?

I heard someone say that it's similar to the difference between modern English and the English of the King James Bible. I would say that that's a fair description. All of the letters and vowels are the same, but there are aspects of the grammar of Biblical Hebrew that are different than that of the modern language. There are also many words that are either not used anymore simply because of the way a spoken language changes over time. However, Modern Hebrew speakers are usually able to understand the majority of Biblical Hebrew without too much difficulty. From personal experience, I can say that I found it quite easy to pick up the Biblical grammar just from reading the Torah (the first five books of the Bible) in Hebrew and studying the modern language on a daily basis. After a couple years of reading the weekly Hebrew "parasha" or Torah portion, only the very poetic parts were difficult for me to understand. There were definitely vocabulary gaps at times, but I would try to look up a couple words every once in a while, and over time I was able to understand just about everything that I read in the Hebrew Torah. Because of this it makes sense to me to learn modern Hebrew, since hearing and speaking a language are two of the most important aspects for learning and retaining language. Most of the use of Biblical Hebrew will be in reading, if you are one of the very studious and scholarly types, you may not have an issue learning a language that isn't spoken, but for many people it is rather difficult to retain the vocabulary without speaking and hearing it. This is why I suggest learning modern Hebrew and supplementing your studies with reading the Biblical language.