In the Force.com platform, the Record ID is an important thing to understand before learning about data management operations. The Record ID is the unique identifier of a record. It is analogous to a primary or foreign key field in a database table. The record IDs can be obtained in the following ways:
URL of the record detail view
Running reports
Web services API
Formulas
Data loader (API Access)
The Force.com platform generates the record ID when a new record is created. There are two forms of a record ID:
15-digit case-sensitive form: This form is used in the URL and report. The reports framework does not expose IDs for all objects.
18-digit case-insensitive form: This form is used in Data Loader. When you migrate data from legacy systems or from a comma-separated values (CSV) file or export data to a CSV file, you have to use an 18-digit format. An API always returns the record IDs in an 18-digit format, but APIs will accept either the 15-digit or 18-digit format.