Indian nationals travelling abroad require a valid Indian passport and travel authority for the destination country. The travel authority is normally in the form of Visa, which is obtained prior to the journey, except in case of countries where "Visa on Arrival" is available.
Indians travelling abroad may also note that some countries insist for certain minimum period of passport validity for allowing entry & for this confirmation should be obtained from the Embassy/travel agent.
Indians having the ECR category of passports require POE clearance from Ministry of Overseas Indians Affairs for certain destinations. For further details see separate topic on ECNR / ECR / POE.
Even for re-entry into the country, an Indian national is required to be in possession of a valid Indian passport or travel document issued by Govt. of India.
PCC: Indian nationals may obtain police clearance certificate from the concerned police station of their place of residence in India OR the Concerned Regional Passport office (RPO).