The Neue Photographische Gesellschaft (NPG) was the first and biggest early producer of real photograph postcards, using a rotary printing method. Arthur Schwarz (1862-1944) founded the company in 1894 in Berlin-Schöneberg, and there were later subsidiary companies in other countries. The company closed in 1921, although the NPG trademark and the postcard department were acquired by E. A. Schwerdtfeger and ran until WW2.
NPG was in the cartels P.R.A. and N.B.C. It was probably the prime mover in both these cartels.
NPG having been so big, there is plenty of information about it on the web. Here is a (German language) site dedicated to NPG. Here is a good account in English (TPA).
See also the entry here for La Compagnia Rotografica, Italy.
NPG as was common at the time sold sets of cards, six cards becoming the norm, in packets. An image below shows a packet sold via E J Hey & Co, which was the sole agent for NPG in the the UK from about 1908 until the war stopped things. Popular in the UK were cards of children and particularly birthday cards.