Every record within Salesforce is assigned a record ID when created. This ID is unique throughout your Salesforce org, and is referenced frequently when performing operations using existing Salesforce data.
15 Digit Record ID
To find the record ID within the user interface, simply navigate to the record. The ID will be shown as part of your URL, and is 15 digits in length:
You may see variants of this in configuration, such as viewing record types. Simply click on the record type of a particular object, and you can easily locate the id of that record type:
18 Digit Record ID
The API (e.g. Data Loader) will accept 15 or 18 character IDs, but will return 18 character IDs by default. The 18 character ID is a combination of the 15 character ID with 3 characters added to ensure that it is unique on a case insensitive basis (this is to support legacy applications). Truncating the last 3 characters from the 18 character ID will result in the 15 character ID. The CASESAFEID function case can be used to derive the 18 digit ID within a formula field.
|15 Digit ID||18 Digit ID|
|Unique (Case Insensitive Basis)||No||Yes|