SRD:Artisan's Tools

Artisan’s Tools
These special tools include the items needed to pursue any craft. Without them, you have to use improvised tools (–2 penalty on Craft checks), if you can do the job at all.

Masterwork
These tools serve the same purpose as artisan’s tools (above), but masterwork artisan’s tools are the perfect tools for the job, so you get a +2 circumstance bonus on Craft checks made with them.