what exactly is the difference between a social worker and a Psycologist?

A social work worker usually works within the confines of a “service agency” most commonly ran by the Government. Their jobs are not exclusively mental health.

A Psychologist works exclusively in the metal health industry, most often without the ability to dispense medication which requires another degree and more education. Most commonly Psychologists stick to therapy, group and individual.

And for the record, a psychiatrist had the extra degree and education and can dispense medication.

Anymore, it depends on the training that a social worker has received. Social workers are no longer only government agencies. Many of them are very specialized. In fact, many insurance companies more readily seek out reimbursement for therapy with Social Workers than Psychologists, as sometimes the hourly fee is not as expensive. Social workers can definitely have specialized clinical capabilities... therapeutic or otherwise. There are thousands... probably hundreds of thousands of Social workers in the country who strictly have Therapy practices. If you find a therapist who is a social worker, vs a psychologist, it doesn't necessarily mean much as to their capabilities... it's more a statement as to which course of 'degree'-ing they chose.

All that GoldGirl said is correct.
Here's a bit more info: Choosing a Therapist