Variables, classes, and references

I just learned about references today (a bit late, but yeay!)

I wanted to double check, if you have a complex class (not a standard number or string) will it be passed as a reference, or will it ALWAYS be cloned every time you do a standard “assignment”? Or will it pass a reference like it does in AS3?

Is there any standard naming convention for variables that are references rather than direct values?