Instance means...

In oracle The simplest description of instance : combination of memory and process architecture

I sql server, what is the Instance.? it is combination of memory and process architecture if yes then pls give name those processes ?

A combination of menory and process architecture evidenced by a completely unique 'instance' of SQL Server. A computer can contain multiple SQL Server Instances, each with its own databases, security, users.|||

you can install sql server services more than one occurence (instance) in a same physical machine. If you install many sql server services on a single machine, you said to have many instance of sql server. One instance can be Default instance(which will have physical machine name as instance name) and all other instance can be called as Named Instances. Each instance run in different process and memory are and listen to different port. There are sql server serices which is instance instance aware and instance unaware.

eg. Database engine service is INstance aware where as SSIS is not (SSIS is common service for all instances)

Madhu