Why do table names in SQL Server start with "dbo"?

If you are using Sql Server Management Studio, you can create your own schema by browsing to Databases - Your Database - Security - Schemas.

To create one using a script is as easy as (for example):

CREATE SCHEMA [EnterSchemaNameHere] AUTHORIZATION [dbo]

You can use them to logically group your tables, for example by creating a schema for "Financial" information and another for "Personal" data. Your tables would then display as:

Financial.BankAccounts Financial.Transactions Personal.Address

Rather than using the default schema of dbo.

dbo is the default schema in SQL Server. You can create your own schemas to allow you to better manage your object namespace.

It's new to SQL 2005 and offers a simplified way to group objects, especially for the purpose of securing the objects in that "group".

The following link offers a more in depth explanation as to what it is, why we would use it:

Understanding the Difference between Owners and Schemas in SQL Server