Why Do Record IDs Contain 15 Characters in Salesforce and 18 in the Destination?

Salesforce represents records with a 15-character, case-sensitive ID in the native platform. However, to avoid issues with case-insensitive applications, all calls to Salesforce's API return an 18-digit, case-safe record ID. As a result, the same record may appear with a 15-character ID in Salesforce and an 18-character ID in the destination.

See Salesforce's Field Types documentation for more information.