I thought a diploma is the tangible certificate that shows you acquired a degree in something? Like, the degree is the course of your education.
Not really. A tangible degree (certainly in Britain and Commonwealth countries, and I suspect everywhere else) is a physical document, a certificate, which historically was inscribed on parchment, these days probably done on a laser printer. It is issued to a successful student at then end of a course of study (his or her degree course). Usually graduands are offered the choice of receiving their degree certificates at a formal ceremony at which they wear academic clothing and headgear. Very often the degree certificate is rolled up, with a band or ribbon around it.
Here is a picture of a student who has just received his degree:
This one is rolled up:
"Diploma" can be a global term covering all educational qualification certificates of varying grades, but in certain educational systems it can be a lesser qualification than a degree, often vocational, or a greater one. In the European education system a Diploma in Engineering ("Dipl. Ing.") is a prestigious qualification superior to a first degree, awarded at the end of a four-year course.
Of course, degree (certificate) is tangible, whereas a degree (qualification) is not. The first is proof of the second, although if the certificate is lost or destroyed it doesn't matter, since the awarding institution will keep a record.
Furthermore, many people use the word 'degree' to denote the entire course, e.g. "I'm starting a degree [course] next year".
So three things: qualification, certificate, course.