What is Frankincense?

What will save the frankincense tree? Daily Telegraph
What Does Selling the Three Wise Men - Gold, Silver, and Platinum -... Zacks

HowStuffWorks "What are frankincense and myrrh?"
The three wise men in the story of Christmas carried gold, frankincense and myrrh. What are frankincense and myrrh? Can you still get them these days?