Certification means that the dog has been tested and shown to meet certain minimum standards.
Most countries only recognize service animals from approved programs. In those countries the programs certify their own dogs.
There are no standards or procedures for certifying a service animal under U.S. federal law. Certification is not required as a condition of using an animal as a service animal. However, the person using the animal must meet the legal (not medical) definition of "disability" and their dog must be individually trained to perform tasks that mitigate the owner's disability. They must also have sufficient training to behave appropriately in public (no barking, making unwanted contact with other members of the public, or disrupting business by misbehaving). Service animals who pose a direct threat to others by growling, lunging, or otherwise menacing people can be barred from public access.
Fake certification is for sale over the Internet. You can check whether a certificate is from a legitimate service dog program or a scam business selling fake certification by doing a Google search on the name of the certifying agency. If it's a scam, it will be apparent from a quick review of their website because they will sell their certification to anyone for a fee without ever actually training or evaluating the dog themselves. These organizations prey on the disabled, selling them something they don't need for $40-$250 that they could produce at a copy center for under $5 (if they did need it, which they don't). They are a haven for pet owners wanting an easy way get a pet into motels, on planes, or to take Fifi shopping on a lark. These businesses do a great disservice to real service dog teams by bluffing business owners into accepting ill-behaved pets as trained service animals and by taking money out of the pockets of the disabled themselves. These fakers in turn diminish the reputation of real teams by behaving inappropriately.
Real service animals don't need certification. A business may verify an animal is a service animal by asking whether it is required because of the person's disability and what the dog is trained to do to mitigate that disability. They may ask this regardless of whether a dog is "certified," and an owner who refuses to answer can be barred from the facility.
A license is something that all dogs are required to have. Individual states, counties or cities may provide licenses in accordance with their own laws or ordinances. Service animals are not exempt from any licensing requirements of local authorities. If dogs residing inside the city limits are required to wear a city license tag, then this also applies to service dogs. In some states, counties, or cities, special service animal licenses are available in lieu of a regular dog tag, but they cannot be required as a condition of access. Some localities also waive the licensing fees for service animals, but this varies.
Service animal registration is a scam. It is a for profit business. It's purpose is to make money at the expense of gullible people with disabilities and those who just want it easier to break laws. Registration means nothing because the dog is never evaluated, never even seen by the agency issuing the registration. It's just a piece of paper that any idiot can buy for between $40 and $250 dollars and that could just as easily be printed on a home computer for a few cents. Registration scams exist primarily to help pet owners pass off their pets as service animals so they can get them on airplanes, into motels, and into stores with them. Real service animals don't need this kind of registration.