A certificate is awarded to those that have completed an educational or vocational program to gain knowledge and skill in a particular industry. Certificates may or may not require an examination to be awarded. They are voluntary and awarded by non-government organizations such as professional associations or companies. Certificates are used to show that a person is competent and has met industry standards to perform the job required.
A license is basically a mandatory certification. They are normally issued by the government and gives a person legal designation and authority to practice within their profession and scope of practice. Only those licensed are legally allowed to perform the set and clear requirements of the profession.
Sometimes the terms licensure and certification are used interchangeably by state agencies, usually when governments require certification before licensure and utilize private organizations to conduct these certifications. Despite the terminology used, the bottom line is that if an occupation has a defined scope of practice, and only those given authority to practice comes from governments of the state, then they are considered licensed in a sense where there are legal requirements to practice in the profession.