Record Id

There are two versions of every record Id in salesforce :


15 digit case-sensitive version which is referenced in the UI
18 digit case-insensitive version which is referenced through the API

• 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 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.   Salesforce.com recommends that you use the 18-character ID.
• The last 3 digits of the 18 digit ID are all capital letters and each of those 3 is a checksum for the other digits in the 15 digit ID. The 16th digit is a checksum of the first 5 characters, the 17th checks the second 5, and the 18th checks the last 5.