Certificate of identity

A certificate of identity, sometimes called an alien's passport, is a travel document issued by a country to non-citizens (also called aliens) residing within their borders who are stateless persons or otherwise unable to obtain a passport from their state of nationality (generally refugees). Some states also issue certificates of identity to their own citizens as a form of emergency passport or otherwise in lieu of a passport. The visa requirements of certificates of identity may be different from those of regular passports.
Types.
1951 Convention documents (for refugees).
A certificate of identity issued to a refugee is also referred to as a 1951 Convention travel document (also known as a refugee travel document or a Geneva passport), in reference to the 1951 Convention Relating to the Status of Refugees. 145 countries are parties to the 1951 Convention and 146 countries are parties to the 1967 Protocol Relating to the Status of Refugees. Notably, the United States is not a party to the Convention, but provides travel documents to its lawful permanent residents, either as a Re-entry Permit or a refugee travel document under the 1967 Protocol.
1954 Convention documents (for stateless persons).
A certificate of identity issued to a stateless person is also referred to as a 1954 Convention travel document, in reference to the 1954 Convention Relating to the Status of Stateless Persons. 89 countries are parties to the 1954 Convention.
Unlike a refugee travel document, a certificate of identity issued by most countries does not in itself entitle the holder to readmission into the country.
Non-Convention travel documents.
Non-Convention (or non-National) travel documents are travel documents issued by a country to non-citizen (also called alien) residents who do not have access to passport facilities from their own countries, are not recognized as either Convention refugees, and are not officially stateless under the 1954 Convention relating to the status of stateless persons (or the country they live in has not signed that convention). 
In these cases there is no formal international agreement to regulate the issue of travel documents to these people although most countries will issue their own version of a non-convention travel document to residents. These documents broadly meet ICAO standards for international identity documents. They are known variously as Alien's Passports in mainland Europe and Scandinavia and as a certificate of identity in the United Kingdom, Australia and Hong Kong.
National examples.
Certificates of identity are issued under various names, including: