The book of Proverbs, and several other places in the Bible, mention the "fear of the Lord".
Proverbs 9:10 (ESV)
The fear of the Lord is the beginning of wisdom, and the knowledge of the Holy One is insight.
In other contexts, such as when God spoke to Abraham, he said "fear not".
Genesis 15:1 (ESV)
After these things the word of the Lord came to Abram in a vision: “Fear not, Abram, I am your shield; your reward shall be very great.”
In yet other contexts, the "fear of the Lord" is praised as a great thing.
Isaiah 33:6 (ESV)
and he will be the stability of your times,
abundance of salvation, wisdom, and knowledge;
the fear of the Lord is Zion's treasure.
What is the difference in these context and in what sense are Christians supposed to "fear" the Lord?