15 and 18 digit id in salesforce

As we know that each record Id represents a unique record within an organisation. 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

The last 3 digits of the 18 digit ID are a checksum of the capitalizations of the first 15 characters, this ID length was created as a workaround to legacy systems which were not compatible with case-sensitive IDs.
The API will accept the 15 digit ID as input but will always return the 18 digit ID.