What is the difference between abnormal vaginal bleeding and dysfunctional uterine bleeding?

Not much : Abnormal bleeding can be caused by tumors, polyps, lesions, cancer, etc & has an etiology (cause) identified, ie - "abnormal bleeding from fibroids". Dysfunctional bleeding usually doesn't have an organic cause and is attributed to hormonal imbalance. One's a description (sign), the other is a diagnosis. I hope that helps a little. They are also used interchangeably, to further confuse you...

Hormonal : Dysfunctional uterine bleeding is defined as abnormal bleeding due to hormone imbalances. Abnormal uterine bleeding is a broader, more general term including hormone imbalances as well as organic causes like fibroids, polyps, adenomyosis, arteriovenous malformations, and mullerian anomalies.