Just as there are different denominations in Christianity, Shiites represent a population of people that follow Shi'a Islam. In terms of population, this denomination is the second largest in Islam. Shiites are known to be a group of people that follow the teachings of Prophet Mohammad and saw Ali as the right person to be called the first imam after Prophet Mohammed, including some successors of Ali.
There are other sects that exist within Shiites. Some are called the Twelvers because they believe there are only twelve imams after Prophet Mohammed, and Ali was one of them. On the other hand, Kurds represent a group of people that are similar in terms of race, culture, who inhabit some parts of Iran, Turkey, Iraq, Russia, etc. Kurds represent an ethnic group, and it is not a religious group like Shiites. Within these people, there are some who belong to Shiites, and there are others who belong to other denominations that exist within Islam.