It is used for securing the location of the pin that is inserted into the fork. It contains tapered holes to accommodate taper pin. In a universal joint, two collars are used to secure two pins.
It holds the pin and collar in the desired location. It prevents the pin from rotating or moving. This ensures the rigidity of the universal joint. An image of a taper pin is shown below:
Note: Taper pin is usually smaller in size. For illustrative purposes, the taper pin shown in the image above is larger.
Shaft is the rotating machine element that is to be connected by the universal joint. It is cylindrical in shape. It contains a keyway to accommodate shaft key. You can understand this by taking a look at the following image:
Note: Shafts are usually longer in size. For illustrative purposes, the shaft shown in the image above is shorter.
It is used to make the universal joint rotate with the shaft. It makes the shaft to properly fit itself in the universal joint.
Shaft key might be straight or tapered. The key shown in the image below is tapered.
For a complete understanding of the assembly of a universal joint, read this article.