Public sector companies are among the most sought-after employers in India, but "PSU" covers very different organisations.
Central PSUs
Owned by the Government of India through a ministry: ONGC, NTPC, SAIL, BHEL and hundreds more. They recruit nationally.
State PSUs
Owned by a state government: power distribution companies, transport corporations, state industrial bodies. They mostly hire within the state.
Public sector banks and insurers
Recruit through IBPS, SBI or their own exams rather than GATE.
Where to start
Decide which group fits your degree and location, then follow those companies' career pages.
A sorted guide to PSU in India with status categories is on govtpvt.com.