Licensure is where a governmental agency grants a person legal permission to carry out an act. Licensure frequently, but not always, involves certification. Once a license exists for a certain activity, it usually becomes illegal to carry out that activity without having that license.
Certification is a process whereby some agency declares that a person, an item, or process meets some set of standards. Sometimes a certifying agency is the government, sometimes it is a private company. A certification is only worth anything if everyone can believe that the standards are actually being enforced.
When a person graduates from high school, they receive a diploma. A diploma is a certification from the school board that declares the person whose name appears on that diploma has met the standards of a high school graduate. When a person shows you a high school diploma, it is supposed to mean that the person can do certain things: read, write, perform mathematical calculations to a certain level, those sorts of things.
You will note that I said SUPPOSED to mean. There are so many exceptions, diplomas haven’t meant anything for decades. A diploma doesn’t even mean that you were there for class for the majority of the time.
Now California has given up even pretending that a high school diploma means anything. Since they have declared that students can not receive a D or an F, every student, no matter how incompetent, will be at least a C student. A California high school diploma is worthless, and now the powers that be have admitted that.
The problem here is that colleges and employers nationwide will accept the diplomas, and the unearned grades attached to them. A high school diploma in California is now no harder to get than a fishing license.