A Salesforce ID represents a unique Salesforce record. The 15 character ID comes in an alphanumeric format i.e. 0019000000CHAus.
A simple way to retrieve this is via the URL.
A Salesforce record ID comes in an 18 character format i.e. 0019000000CHAusAAH.
The 18 character ID is similar to the 15 characters mentioned above but appended with 3 additional characters.
The 18 character ID is only retrievable through the Salesforce API using a web service query or an Integration tool, Data Loader or Apex Explorer. Here is an example response when querying for Account records in Salesforce:
<?xml version=”1.0″ encoding=”UTF-8″?>
These 18 and 15 character IDs represent the same Salesforce record retrieved via different mechanisms. When integrating Salesforce or using the Salesforce API, utilizing either the 15 or 18 characters ID is supported for querying or updating a specific record.