What's the difference between CSR and Marketing/PR? When companies do CSR and when do they do marketing/PR?

CSR is not an activity which aims ultimately to market the company or its products. it is an attempt made by the company as a part of its human-side activities. As all are aware, marketing-related public relations are for ensuring the growth and brand image of the company. Ultimately, both will ensure the enhanced goodwill of the company. 

The difference is in the interest of the company of which customers it needs to focus on. To increase customers vertically (i.e., in many and different fields), company do CSR. To increase customers horizontally (i.e., in a specific field) company do Marketing/PR. Thanks

The key difference whereby effective communication methods are largely absent from the social responsibility literature; yet by including such techniques, one can enhance the development and overall impact of managing corporate–stakeholder relationships.