BASIC UNDERSTANDING OF SQL JOIN

there are multiple joins out there, we can group those into INNER JOIN, OUTER JOIN, FULL JOIN. This joins may be simple to understand. But the problem comes from the point of how we are joining. For example I have a table customers and students. Yeah there is no relationship between a customer and student in this example. Can I join this customers and students tables. If you say `no` then the understanding may have to cleared. Basically how we are writing joins