The Small Spec Bearing is for small bearing compatible yo-yos, like the Velocity.
The Large Spec Bearing fits the majority of all YoYoFactory yo-yos.
The SPEC–X is an upgrade to the Spec bearing, and is built tighter and more robust for an extended bearing life. It has a very short break-in period, tends to make less noise, and is less prone to failure or lockups due to impact or lack of lubrication.
Center Trac Bearings are an upgrade that allow the string to remain trapped in the center away from hitting the side of the yo-yo during instense string trick play. This will make your yo-yo as nearly unresponsive as possible and will make string play smoother as it keeps the string away from catching the side of the yo-yo.
Generally speaking all Bearings do go through phases of break-in where the noise of the bearing will change over its life. Bearings do need to be maintained and are designed to operate with some lubrication. While they can be run dry performance may be compromised in the long run.