How to change the center of an object? And why an object is called a link?

Hi there, new to gazebo, trying to create a joint between 2 basic objects, one cube and one cylinder. However, the center of an object is by default at the geometrical center, whereas I want the joint to be created at a different place. in particular, I want the joint be attached to one end of the cylinder. But I don't know how to do it. I thought I should change the center of the object first, but I failed to find the option to do it. Also I can't understand why an object is called a link in Gazebo, is it because they are just the bonding box? and I'm supposed to put a more complex shape inside of it? Thanks.