NPT and BSP are the two most common pipe thread standards in the world, and they cause more headaches than almost any other fitting decision. The threads look nearly identical, the sizes are labelled the same way, and they'll often connect with each other just enough to feel like they're working right up until they leak.
Here's what's actually different between them.
The Thread Angle
NPT (National Pipe Tapered) uses a 60° thread angle whereas BSP (British Standard Pipe) uses 55°. That 5° gap means the flanks of the threads don't properly contact each other when you mix the two. The fitting will connect, you'll feel resistance as you tighten, and it'll seem fine. But you're not getting real metal-to-metal contact, and no amount of PTFE tape will make up for that in a pressurized system.
The Thread Pitch
Pitch is how many threads fit in an inch. At 1/2" nominal size, NPT and BSP actually share the same pitch at 14 threads per inch, which is part of why they cross-thread so easily and why that size causes the most confusion. At other sizes, the pitches diverge, and the fittings simply won't connect properly at all.
The Taper Rate
Both standards are tapered meaning the diameter increases as you move along the fitting, creating a wedging action when tightened. NPT tapers at 3/4" per foot and BSP tapers at 1:16.
How to Tell Which You Have
The easiest approach is to check the documentation that came with the fitting or equipment. When that's not available, a thread gauge will tell you definitively. You can also use country of origin as a rough guide, European and UK equipment almost always uses BSP whereas North American equipment uses NPT. That breaks down quickly with imported goods and online purchases, so if you're not sure, check before you buy.