NPO refers to the dielectric composition used.
The biggest difference it makes is in temperature coefficient, but the actual values are also closer tolerance to stated values. They're the standard in high precision circuits.
The other two main ones you see in ceramic discs are X7R and Y5V. X7R is what you use for the majority of ceramic disc applications. Y5V is the cheap junk- avoid it if you can.
I go for silver-mica myself for small values.... no piezoelectric properties to the dielectric.
Thanks, Peter (and Paul)... I have the Tab book, "Guide to Understanding Electricity and Electronics". Trouble is, I just don't keep it here at the office where I do my online ordering. :oops: I was doing a little reading ahead last night and practically dropped the book open to the page that discussed this very topic. (things that make you go, "Oh-hhh")
In a nutshell, my understanding is that NPO (Negative-Positive-Zero) refers to the capacitor's ability to avoid a change in value with regard to temperature. In this case, an NPO hangs pretty close to it's value over a wider range of temperatures.
I drifted off momentarily, trying to envision an application that would -want- a cap's value to fluctuate by means of temperature variation but I'm thinkin' that will take a little more reading.
Great book for a newbie, btw. I highly recommend it if you take the time to slog through the math and get a grip on the concepts. A month or so ago, I -never- thought I'd be hip deep in this world.