SQL Server - Server name same as the instance name and most common use for instances? Is the server name the same thing as an instance name? I have looked around and am not sure; I think they are but I don't just want to guess. 

Also, what is the most common use for SQL instances? I understand why it would be useful to have different instances, but I would like to know what others find the most use of it from.
Servers are like the physical location. Instances are the actual SQL Server functionality. They are what you connect to in order to use SQL Server.


Each Instance belongs to only one Server. 
You can have multiple instances per Server (so T_COMPUTER\SQLEXPRESS and T_COMPUTER\SQLSERVER could be 2 instances on the same Server).
An Instance is a fully self contained SQL Server installation.
You can have a default instance configured, which is the Instance accessed when you connect to the Server directly (i.e. T_COMPUTER, actually connects to T_COMPUTER\INSTANCE)