lens mount are exclusive to each brand of camera. the size is different and the point for electrical contact is also different. if you have a nikon, you need a NIKON MOUNT lens ( though not necessary nikon lens cos 3rd party like tamron, sigma etc also produce lens w mount for nikon, canon etc.
however, there are ways to 'cheat'. you can get adaptors that will ' change the mount of canon to fit a nikon lens for example. thought the mount does fit, the lens wil not be able to 'communicate' with the camera as the electrical contact will still be different.
tamron used to solve the problem during the Manual focua era by making lens and selling a suitable adaptor mount depending and what camera you have. but when it comes to the AF era, this method wont work anymore cos of the electrical contacts