drill-down means analysing the in different levels going to 
the down let us eg as year--->quarter,quarte---> month and 
so on
drill-up means analysing the in different levels going to 
the up let us eg as month--->quarter,quarte---> year  and 
so on

Suppose i have set the hierachy in the universe as 
Country->State->City->Region

Drill down and Drill up= I have a report which shows country
and revenue,when i do drill down i will the report as State
and revenue,further i will get city and revenue.But in case
of drill up ,when i say drill up from city,revenue i will
get state and revenue and goes on.

Drill By-You can select the object through which you can
apply the drill.

Drill Through-Suppose i am drilling down by
country->state,suddenly i want to drill on drill hierarchy
say i want select sales manager from the other class.