What is the difference between 15 digit and 18 digit id in Salesforce ?

In Salesforce, whenever user create any component or record then sales force will generate an unique id with which user can identify the record or component.
After creating the record, in the URL user can see the id of the record which is of 15 digits length.
Through user interface user always see 15 digit ids which are Cases-Sensitive.
If the user queries the existing records from the database through API, it will always return 18 digit ids which are Case-Insensitive.
Last 3 digits of the 18 digit represent checksum of the capitalization of 15 digit id.
Based on the first 3 digits user can identify the object of the record.
All the record belongs to same object will contain same first 3 digits.