<p>I have heard umpteen times that we 'should not mix business logic with other code' or statements like that. I think every single code I write (processing steps I mean) consists of logic that is related to the business requirements..</p>

<p>Can anyone tell me what exactly consists of business logic? How can it be distinguished from other code? Is there some simple test to determine what is business logic and what is not?</p>

Simply define what you are doing in plain English. When you are saying things businesswise, like "make those suffer", "steal that money", "destroy this portion of earth" you are talking about business layer. To make it clear, things that get you excited go here.
When you are saying "show this here", "do not show that", "make it more beautiful" you are talking about the presentation layer. These are the things that get your designers excited.
When you are saying things like "save this", "get this from database", "update", "delete", etc. you are talking about the data layer. These are the things that tell you what to keep forever at all costs.