That is a tough one. Generally it is used to distinguish what thing we are talking about.
The cat walked away.
instead of
cat walked away.
or
What did he do to the cat
instead of
What did he do to cat?
If you had a specific cat named fritz, then there is no need for the "the"
Fritz walked across the street.
this is since we already know who fritz is...
Here is a good definition.
http://www.bartleby.com/61/47/T0144700.html
The cat walked away.
instead of
cat walked away.
or
What did he do to the cat
instead of
What did he do to cat?
If you had a specific cat named fritz, then there is no need for the "the"
Fritz walked across the street.
this is since we already know who fritz is...
Here is a good definition.
http://www.bartleby.com/61/47/T0144700.html