Baseband: control the GSM frequency. This is how Apple lock iPhone to stop people use it under other GSM network rather than AT&T. Due to T-mobile has similar frequency, locked iPhone can be used under T-mobile some how. The latest baseband 5.12.01 hasn’t been unlocked yet (except some videos on YouTube), so hacker need to downgrade it to 5.11 to unlock iPhone. However, the latest bootloader 5.9 has fix the bug which allows people downgrade baseband from 5.12.01.
Bootloader: it is a hardware, can’t be upgrade/downgrade unless modify the iPhone hardware, just like the camera. Only the bootloader before 5.9 has the bug to allow people downgrade baseband. However, most of the new iPhone 3G/3GS is using the bootloader 5.9.