What is a co-related sub-query and how it is different from nested queries ?

A correlated subquery is a special kind of subqery that allows reference to specific rows in the principal query.

select title_id
from titles
where exists (select * from sales where sales.title_id = titles.title_id)

That query correlates sales to a title_id. You don't get a row if there's no sales. The " = titles.title_id" is what handles the correlation.

Let's say you had some trade data:

CREATE TABLE dbo.TradeData (Symbol varchar2 (20) NOT NULL ,
  dateTraded datetime NOT NULL ,
  TradeType varchar2 (50) NOT NULL ,
  Signal int NOT NULL ,
  priceTrigger decimal(18, 5) NOT NULL)

Now, we want to see the latest trade for each symbol and tradetype:

select  sd.*
from    TradeData sd
where    sd.datetraded = (select	max(dateTraded)
    		from	TradeData sd1
    		where	sd.symbol = sd1.symbol
    		and	sd.TradeType = sd1.TradeType)
order by
    sd.symbol, sd.strategy

The subquery correlates back by rows specific to that symbol, to allow you to find a row with a specific property for eacy symbol retrieved. The max(dateTraded) is computed for each symbol and TradeType and only the dateTraded with the max date is allowed into the final result set. I usually do that kind of query this way:

select  sd.*
from    tradedata sd
inner join (select max(datetraded) as datetraded, symbol, tradetype from tradedata group by symbol, tradetype) msd
on  sd.symbol = msd.symbol
and sd.tradetype = msd.tradetype
and sd.datetraded = msd.datetraded)

This isn't necessarily better but it shows a different way of doing it. In fact, it just happens to be the first way I figured out to solve such a problem, so it's my habit now. The inline-view, msd, yields a list that can be joined back for the latest datetraded for each type and symbol.

(These are from SQL Server examples... might need editing for Oracle but the idea works the same way)