Clarification of notation

[Bruce Whealton]

Hello all,
          I recently started learning python.  I am a bit thrown by a 
certain notation that I see.  I was watching a training course on 
lynda.com and this notation was not presented.  For lists, when would 
you use what appears to be nested lists, like:
[[], [], []]
a list of lists?
Would you, and could you combine a dictionary with a list in this fashion?

Next, from the documentation I see and this is just an example (this 
kind of notation is seen elsewhere in the documentation:

str.count(sub[, start[, end]])
This particular example is from the string methods.
Is this a nesting of two lists inside a a third list?  I know that it 
would suggest that some of the arguments are optional, so perhaps if 
there are 2 items the first is the sub, and the second is start?  Or did 
I read that backwards?
Thanks,
Bruce