FSC stands for the Forest Stewardship Council which is the most well established association that certifies sustainable lumber. FSC certified lumber is sourced from a sustainably managed forest.
I strongly encourage all consumers and constractors to buy FSC lumber whenever possible. When you buy FSC Lumber you can be assured that the lumber was sourced from an accredited organization and from a forest that is managed in a more sustainable manner.
I’ve spoken with a number of industry professionals about the question of sustainable lumber, and the consensus is that FSC certified products are the best option and they are widely available.