ID fields in the Salesforce.com user interface contain 15-character, case-sensitive strings. Each of the 15 characters can be a numeric digit (0-9), a lowercase letter (a-z), or an uppercase letter (A-Z).Two unique IDs may only be different by a change in case.
Because there are applications like Access or Excel which do not recognize that 50130000000014c is a different ID from 50130000000014C, an 18-digit, case-safe version of the ID is returned by all API calls. The 18 character IDs have been formed by adding a suffix to each ID in the Force.com API. 18-character IDs can be safely compared for uniqueness by case-insensitive applications, and can be used in all API calls when creating, editing, or deleting data.
To convert 18-character ID to a 15-character version, truncate the last three characters. Salesforce.com recommends that you use the 18-character ID.
- 15 digit case-sensitive version which is referenced in the UI ( at Database level).
- 18 digit case-insensitive version which is referenced through the API.