High level: Is like the overview of any test/ scenario, in 
which you define the major parts of your scenario. 

Exp: Suppose you are given with a module, for coding.In 
that case first you will decide the major things like 
Language, Database, SystemCong. etc... then proceed further.

Low level: It's like "Exploratory Testing" in which you 
study the whole thing in detail.

Exp:If you take the same example... After the major thing. 
Now you are ready for coding.So here you will decide some 
variable names and use some std's for coding. so that any 
one can understand , what have you done.......

Yahoo mail example
High Level: Successfull login user can send a mail
Low level: System should provide a text box to enter "To" 
Address.