The "International Certificate of Vaccination or Prophylaxis (ICVP)" (or "Yellow Card") is mainly required for proof of vaccination when going to or from countries that are at risk for Yellow Fever. However it is all used for recording all of your travel immunizations. It is available from your physician or health department.
The "Yellow Card" as it is commonly refered to, is a small yellow document (imagine that) designed to fit into a passport. When you get an immunization, the health specialist will fill out and sign this card. The entry must be validated with an official stamp by the health specialist. It is approved by the World Health Organization as the official document verifying that proper procedures were followed in administering vaccinations for foreign travel and is used to demonstrate receipt of required vaccinations for entry into foreign countries.
About the only thing this is currently used for is checking for Yellow Fever vaccination when coming from an infected areas. Very rarely some border guards may also check for a Cholera vaccination in some third world countries.
The International Certificate of Vaccination is used to record the following information:
In the case of Yellow Fever, the International Health Regulations allow countries to require proof of vaccination for entry and/or from travelers arriving from infected countries (30). Travelers arriving without a completed ICVP may be quarantined or refused entry unless submitting to onsite vaccination. Vaccines must be approved by WHO and administered at an approved yellow fever vaccination center. You should receive a completed ICVP, signed and validated with the center’s stamp where the vaccine was given.
Yellow fever vaccination must be given at a certified center in possession of an official “Uniform Stamp” which can be used to validate the ICVP. The ICVP must be validated by the center that administers the vaccine. Most city, county, and state health department's travel clinics, as well as private travel clinics and individual health-care providers are designated sites. Information about the location and hours of yellow fever vaccination centers may be obtained by contacting local or state health departments or visiting CDC’s Travelers’ Health website at http://www2.ncid.cdc.gov/travel/yellowfever/. An ICVP must be validated to be acceptable to quarantine authorities. If not correctly validated a traveler may have to be revaccinated, quarantined, or denied entry.