They vary, but think the very hottest is called a Phaal. Never had one, and never seen one advertised on a menu. I suspect it may be a bit specialist. From the mainstream, goes from Vindaloo, very hot, through Madras, Jalfrezi etc which are hot, mediums like Samber or Rogan Josh down to mild like Biryani or Bhuna.
The wonderful thing about Indian food is that the above will be on the menu anywhere, but there are so many other regional and cultural variations that you will always come across things you've never heard of or tasted before. Heat isn't the be all end all of curry. Needs a kick, but it's the spices and the flavours that make them a thing of beauty.