A certified copy is a copy (often a photocopy) of a primary document, that has on it an endorsement or certificate that it is a true copy of the primary document. It does not certify that the primary document is genuine, only that it is a true copy of the primary document.
A certified copy is often used in English-speaking common law countries as a convenient way of providing a copy of documents. It is usually inexpensive to obtain. A certified copy may be required for official government or court purposes and for commercial purposes. It avoids the owner of important documents (especially identity documents) giving up possession of those documents which might mean a risk of their loss or damage.
It has some similarities to a notarized copy, which is a form used in some countries, and particularly in some States in the USA. A notarized copy is signed by a notary public (not to be confused with a notary in a civil law country).
The certified copy is signed by a person nominated by the person or agency asking for it. Typically, the person is referred to as an authorised person. The person who is authorised to sign the certificate will vary between countries. Sometimes a person is authorised by legislation to do so (for example a court clerk, solicitor, or notary public), but this is not always so. In some countries, for example the United Kingdom and South Africa, identity documents can also be certified by authorised Post Office staff.
A copy of a primary document that is to be used internationally may have to be in the form of a notarized copy rather than a certified copy. A notarized copy may be more expensive to obtain. A copy of a document to be used internationally may also have to comply with special rules - Hague Convention Abolishing the Requirement of Legalisation for Foreign Public Documents.
If the primary document has to be translated, an additional certificate is usually required. For example, a birth certificate in Russian is to be used in an English-speaking country. Typically, the document must be translated professionally and have the professional's certificate of accuracy attached to the translation together with a copy of the primary document. Then, the primary document, the translation, and the certificate of accuracy are photocopied in the form of a certified copy. In the case of a birth certificate in English to be used in Russia, it is possible that a notarized copy will be required.