Is there an meaning of '[[]]' in a list?

[fl]

Hi,
In the previous exercises, I see list:
cc=[[],[],[]]

Then, I can have this:

ccc=[[[]],[[]],[[]]]

I can also have

ccc[0]
Out[158]: [[]]

ccc[0]='o'

ccc
Out[163]: ['o', [[]], [[]]]


I have question: Is there any difference between [[]] and []?
[[]] can have deeper assignment and use than 

ccc[0]='o'


Thanks,