There’s a few commonly used materials. Wood is a traditional choice, but it’s heavy, subject to the ravages of time, slippery when wet, and can conduct electricity. Aluminum, on the other hand, is lighter, more durable over time, and cheaper. However, aluminum also conducts electricity. If you aren’t going to be working near a power source or electric lines, aluminum is likely a good material for your ladder. If you are, then spend more and go for a fiberglass ladder, which has all the strengths of aluminium while also being non-conductive — and therefore safe to use around electricity sources.