What is a katana tsuka?

Excuse me, but could you please elaborate on the concept of a "katana tsuka"? I understand that it's related to the Japanese sword known as the katana, but I'm not quite clear on its specific function or significance. Is it the handle of the sword? And if so, what sets it apart from other sword handles? Additionally, are there any unique characteristics or features that define a katana tsuka, or does it vary depending on the era or style of the sword? Your insights would be greatly appreciated.