It's not wrong exactly, but it's also not the whole story. CUDA cores are just one facet of measuring GPU performance. You also need to look at things like core and memory clock speeds, the size of the memory bus, and even the type of RAM.
What exactly do you want to do? Just guessing, but, it sounds like maybe you are shopping for a graphics card, is that right? If so, tell us what you are looking for and a budget and maybe we can help you more.
CUDA cores allow complex computing to be done on your gpu, for gaming they are mainly used to handle "soft body" physics where you would see larger particles break into smaller particles, your CPU handles the majority of this though, CUDA is specific to Nvidia, and is supported by Microsoft's direct compute. They are more important for accelerating scientific applications, or 3d rendering.