A Salesforce id represents a specific object instance in Salesforce. For some objects the id starts with a specific set of characters (prefix) in the beginning of the id string, see .
You can obtain the Salesforce id in different ways, either directly in the Salesforce application or by using the API.
1. The most common and easy way is to look in the address field of the browser. This provides you with the 15-digit Salesforce id which is also case-sensitive.
2. You can also get the Salesforce id by using the Salesforce API like Apex Explorer or Data Loader. This provides you with the 18-digit Salesforce id which is not case-sensitive. This way it can be processed by Excel without a problem for example.

Use whichever you want, they both work in all scenarios what I know of.
If you for some reason need to convert a 15-digit id to a 18-digit id there's an for that.