What are the variances among 15 and 18 digit record ids in Salesforce?

ID fields in the Salesforce user interface contain 15-character, base-62, case-sensitive strings. Each of the 15 characters can be a numeric digit (0-9), a lowercase letter (a-z), or an uppercase letter (A-Z). Two unique IDs may only be different by a change in case.
Because there are applications like Access or excel which do not recognize that A0A6D000000RzI7 is a different ID from a0a6D000000RzI7, an 18-digit, case-safe version of the ID is returned by all API calls. The 18 character IDs have been formed by adding a suffix to each ID in the Force.com API. 18-character IDs can be safely compared for uniqueness by case-insensitive applications, and can be used in all API calls when creating, editing, or deleting data.
If you need to convert the 18-character ID to a 15-character version, truncate the last three characters. Salesforce recommends that you use the 18-character ID.
If you need to convert the 15-character ID to an 18-character version, you can use the Record ID Converter bookmarklet script here.