it is a three tier architechure 
the tiers are 1)database 2)application 3)web
cognos architechure  is nothing but overal view of cognos

for user convinence they divided in to 3 tiers 
web tier
application tier
data tier

when u want to hide the column based on some condition then 
we will go to the render variable option.

Render variable is something like hiding(rendering) 
anything using a variable(Condition)

As far as i know

Render variales are generally created to conditionally 
render a particular page or chart or list or block etc...

Though the purpose of the render variable is to hide the 
particular item..we can't say that it is use to hide a 
particular item...

Consider a scenario..
You have created two list reports for two countries

List1 is for both country1 and country2
List2 is for country 2 alone

List1 is created from the query1 
List2 is created from the query2

Now u create a conditional string variable called 'Country' 
which uses country name as a parameter 

Now u associate Variable 'Country' as render variable for 
List1 and List2

Now whenever u run the report..when u select country1 then 
automatically the two queries will be rendered to the two 
list and the two lists will be displayed

But when u select Country2 as parameter then only Query2 
will be rendered and the list2 will be displayed in cognos 
viewer window...

Note...

When u use Conditional variable u r just hiding the output 
from the user..but in the backend the two  queries will be 
processed by cognos....hence ur precious time will be 
wasted..

But when u r using the render variable only the Queries 
which satisfies the parameter will be rendered and hence 
the other queries will not be processed in backend by cognos

I hope this helps you

MDX - Is the Multi Dimensional SQL.
Just like we have SQL for Oracle or MS- SQL (which are 
nothing but Relational Data Bases ), The Query or the 
Syntax for Multi Dimension Data Source like CUBE or other 
OLAP data source use MDX.

So the PowerPlay Transfer uses MDX for building the CUBE.

MDX is a multidimensional expression, we can query directly 
to OLAP source by using MDX and SQL is a structured query 
language, where as we can query to the RDBMS source.
When we r dragging the query items into the report area 
automatically MDX generated that query will send to the 
OLAP source directly.

Report view uses the same report specification as the 
source report, but has different properties such as prompt 
values...etc

By using this we can have multiple schedules for the same 
report with the different values.