Bitcoin Q&A: Key Storage Best Practices

What is a hierarchical deterministic (HD) wallet? How are paper wallets different from hardware wallets? Which of the Bitcoin wallets and / or clients (mobile, desktop, hardware) is best for storing coins? Are hardware wallets with Bluetooth secure? Regarding desktop wallets, what happens if your hard drive fails? Should you use different tools for storage of keys versus spending? Should you encrypt a text file for cold storage of keys?