* The profession: "I'm studying Social Work at the university."
* The field of study: "Social Work is a challenging but rewarding career."
* The academic discipline: "The Social Work department is hosting a conference."
However, you would not capitalize "social work" when it's used more generally:
* Describing an activity: "She volunteers doing social work at the homeless shelter."
* In a more informal context: "He's interested in social work but hasn't decided on a career path yet."
Essentially, you capitalize "Social Work" when it's a proper noun, referring to the specific field or profession. But you use lowercase "social work" when it's a more general descriptor.