Salesforce.com Id attribute seems to have a 15 and 18 character value, whats the difference?

Question

When using the SOAP API to work with salesforce.com (SFDC) it seems that the primary key in the underlying database is Id. Well, there seem to be two representations of this value as either a 15 character version or an 18 character version.

I have been using the 18 since it is clearly more specific, but what is contained in the last three digits, that they can be dropped, seemingly?

Does anyone understand what this is all about?

Answer 1

There are 15 characters in Salesforce IDs in the UI. 18 character case-insensitive version is returned using APIs.