NPT (National Pipe Taper) and BSP (British Standard Pipe) are two types of threading standards for plumbing and fittings. The main difference is in the angle and thread profile. NPT has a 60-degree angle and flattened peaks and valleys, while BSP has a 55-degree angle with rounded peaks and valleys. It's important to match the correct type of fitting with your compressor and other equipment to avoid threading issues.