* High school leaving certificate (or high school diploma in many countries, such as the US and Canada): This is awarded upon successful completion of secondary education. It certifies that you have met the minimum requirements of the curriculum.
* Diploma: This term is more broadly used and can refer to various post-secondary qualifications. It can be awarded after completing a vocational program, a shorter associate's degree program, or even certain professional certifications. It implies a more focused and specialized area of study than a general high school leaving certificate.
In short: A high school leaving certificate is a prerequisite for most diplomas, but a diploma is not a high school leaving certificate.