ID is a technical short form of Identifier. The concept of Id is one of the most basic concepts of Salesforce.
In the Salesforce world, record id identifies a row in a table, or a specific record. For Example let’s consider your client Jane Smith who works at SSS Inc. Jane Smith is stored as a Contact, and SSS Inc is stored as an Account in Salesforce. The Contact Id of Jane Smith uniquely identifies her. It is different when compared to other records in that Salesforce instance. Similarly the account SSS Inc has a unique record id.
The URL can be modified to a different record by just changing the Id in the URL.
15 Digit vs 18 Digit IDs
Salesforce IDs can be either 15 digit Case-sensitive (example 0019000000DmehK) or eighteen digit case-insensitive (example 0019000000DmehKAAR). In this example, they both represent the same data.
The APIs (a programmable call to get information) always return eighteen digit record id. The first three characters identify the type of the object. For example 001 is used for Account, and 003 is used for Contact. This is true across all Salesforce instances.
Even user records have an id. For example the following represents the URL for a user in my developer org. Here 00590000000V0CcAAK is the record id for the user.
How to get an organization id
Often when you need to contact Salesforce about your org, or if you need to contact a Salesforce partner, they may ask for your Organization ID (abbreviated as Org id). This is the unique number that identifies your Salesforce instance. In Lightning, in the Setup section the Organization Id of your Salesforce instance is available in the menu Company Settings → Company Information.