Crafting guide is a mod that allows you to see the recipe of all the craftable items made with the 3x3 crafting table. To see the recipes, either make and use a crafting guide item, or simply press "G" ingame. It has several sorting methods, and a search function. If ever you find an item and don't know what it's for, a good first step is to search it in the crafting guide. If you can't find it there, this wiki is a safe bet.