Hazeman,
You might want to be more specific with your question. If someone asked you the same question about rock climbing gear, where would you start? There's too much ground to cover with such a broad sweeping question. Maybe you need to do a little research here and ask a more focused question we can get our minds wrapped around. Let's say you asked about rope. That subject has been discussed about in length here. Better to find the thread and read before asking a repeat question. Possibly read a tree climbing book (Climbers Companion is good) or view the DVD to get a good grounding.