When it comes to being a professional having a tattoo certification is a must. Every state and locality has different requirements as to what they consider adequate, but that also means that your certification may not cross over if you decide to relocate. Most places have at least a basic inspection and question arrangement where the health department or similar will come in and inspect your premises and ask you questions about how you do your job.
A great example of a state that has strict tattoo certification is Virginia. In Virginia there is a 1500 hour minimum state mandated apprenticeship. To even get an apprenticeship you must have completed CPR, first aid and bloodborne pathogen training. If you've gotten the apprentice tattoo certification then you will have to have written proof that you are learning, for example a notebook with observations and suggestions. In addition you will need to have 50 documented tattoos, though there is no requirement for them to be good your mentor may expect you to do more before signing off on your training. Once your mentor deems you ready there is a state board exam which covers everything from color theory to tax information. If you decide to skirt the laws you can be looking at large fines and jail time for being caught. It may seem like overkill but tattooing is essentially a minor surgical procedure and not having the correct training risks both client and artist. The great thing about finishing an apprenticeship like this is that you will be trained to work anywhere even under the harshest certification requirements and it also gives you respect from fellow artists that you really know your trade.
A state that is a good example of poor tattooing certification laws is New Mexico. The only laws governing tattooing in New Mexico are concerning animals, in which case only a vet may do so. Washington DC also has no laws governing tattooing, and it is still illegal in a few stated entirely so obviously there are no laws for tattoo certification there. It may seem like an apprentices dream for anyone to tattoo who wants to but often shops will still expect you to undergo a traditional apprenticeship even if you do not have a state exam at the end. Your mentor in this case might say that they will consider you no longer an apprentice after a year or more of tattooing. This also is awful if you don't plan on living there forever as most other locations will consider you still at apprentice level even if you have been tattooing for several years.
Though there is no universal level for tattoo certification there is still a basic standard throughout the industry that needs to be met for your to be considered a respectable artist. This can be as little as clean conditions with clean work, you don't have to be the best artist but if you can repeatedly produce solid work in a sterile environment you will still be looked upon better than someone who does great work and risks contaminating themselves and clients.