The other writeup is only partially correct.

Nikkor is the name given to camera lenses made by the Nikon camera company. The naming is in line with the fanciful naming of photographic lenses throughout the 19th century, which gave names such as the Hypergon, etc.

Nikkor lenses are very well regarded, and until recently widely thought to be the best lenses available, especially for 35mm photography.

Nikkor 35mm lenses all have a surprising level of backward compatibility built into them, which allows the use of the first Nikkor 35mm lens to be used in the latest Nikon camera, and vice versa. To an extent, that is.

Nikkor 35mm lenses can be divided into pre-AI, AI, AI-S, AI-P, and autofocus models. The most important differences being pre-AI, AI, and autofocus. AI and autofocus differ in the fact that autofocus lenses can be used to autofocus on autofocus cameras, but are otherwise compatible with AI-compatible cameras. Pre-AI, however, uses a different method to synchronize the lens aperature with the camera meter, and need to be modified to work with AI-coupling cameras.

Some cameras have the ability to use pre-AI as well as AI lenses.

Other Nikkor lenses are made for large format photography, and at least a few Nikkor lenses were made for the older, discontinued Bronica medium format cameras.