What you see here is the U-joint main body, the cross … it’s cross shaped. At the end of each cross is a machined area where the cap lives. This is called the trunion or journal. On the right is a U-joint cap. It’s basically a bearing and race that allow the axleshaft or driveshaft to pivot a few degrees relative to the axis of the U-joint. See all those little needle bearings in the cap? If you accidentally knock off a cap and those bearings fall out you are in trouble unless you can get them all clean and back in the cap. Miss one and your cap is gonna fail! Some U-joints have grease fittings on them. Use ’em to add grease frequently if you’ve got ’em.