You are good at making a particular type of group of objects. Like Knowledge, Perform, and Profession, this skill can be taken more than once.
Check: you can make (as per the Craft skill description in the 3.5 PHB) and assess the value of (as per the Appraise skill description in the 3.5 PHB) objects associated with your chosen Craft.
Synergy: 5 or more ranks in a Craft skill related to the object being tinkered with gives you a +2 bonus to Tinker checks. This bonus increases by +1 for every additional 10 ranks (that is, +3 at 15 ranks, +4 at 25 ranks, and so on).