You have Federal Law, State Law, County Law and City Law.
How your products are labeled comes under the FPLA (fair packaging and labeling act). If a state doesn't have a requirement, it is because it is covered under the feds.
US Labeling Requirements (nutshell):
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Above is one of my blank labels. Note that the name of my company is at the top and prominent. Under it around center would be the name of my name. Under that is what my product is and it HAS to say 'soap'. At the bottom right is the weight of the soap in ounces and grams. At the bottom right is my city, state and zip code***.
***A note about this...so long as the physical address of where I make soap is publicly available, I don't have to put my street address. Second note, consult with state and local laws...they may be different.
EVERYONE must confirm to FPLA for the front label. Depending on what you are selling, you can have different rules and regulations. Example...labeling for cereal is different than labeling for motor oil is different than labeling is for soap is different than labeling for cosmetic. Also, different products can fall under different jurisdictions.
True Soap...soap made wholly with oils/butter and lye fall under the jurisdiction of the CPSC. And the CPSC doesn't give a rat's butt unless are marketing your product to children and then there are a different set of rules. But as long as you are make True Soap, you really only have to follow the FPLA. There is NO requirement for True Soap to have an Ingredients Label on your soap, but you're stupid if you don't because I would never buy your soap without knowing what it is made of. It would be like me be buying yarn and only knowing the yardage; makes a difference. Almost every soap maker I know uses cosmetic rules for their Ingredients Label. And whether you want to put what goes in the pot or out of the pot...just make sure you put it in order by weight.
Anything that is a 'cosmetic'...and in soap making/bath & body that includes shampoo, conditioner, shave soap, lotions, lip balms, scrubs, etc falls under the jurisdiction of the FDA...which has new requirements effective January 1st.
Anything that 'heals/cures/helps' falls under 'drug' and again...FDA and you're talking some bucks now because you're products will have to be tested and you'll have to be licensed.
NOW....everything I have mentioned above is FEDERAL. You will need to check with your state, county and city to see what rules and regulations they have for manufacturing and sales. One good lesson learned about this is Katie with Royalty Soaps. They had purchased a home and had built a shop and were making soap when they received a Cease & Desist or face massive fines. Turns out the use of Sodium Hydroxide was prohibited in a residential area.
Because every state, county and city/town is different, I can't tell you specifically where to look up this information. Best advise I can give you is to follow the strictest rules there are and you can't go wrong. Don't think that just because you are making soap in your kitchen and selling at kitschy craft fairs that you are exempt from following the laws. Now the odds are good given the number of artisan/craft soap makers there are, you're not going to get caught, but understand...it only takes ONE customer to complain.