PGDM stands for Post Graduate Diploma in Management. It is a diploma course offered by the institutes that are recognised by the All India Council for Technical Education (AICTE) and which are autonomous and not affiliated to any university. However, accreditation by Association of Indian Universities (AIU) makes an institute's PGDM course equivalent to an MBA. 

An autonomous institute does not have to follow university standards, so it is free to change its curriculum according to industry standards and change in the business environment. The course will prepare you for a senior level position in a company.

Famous PGDM institutes in India are:

IIMs (Indian Institute of Management)

XLRI (Xavier School of Management), Jamshedpur

SP Jain Institute of Management and Research, Mumbai

NMIMS (Banglore/Hyderabad)

Hope this helps.

All the best.