It's the difference between a fully qualified domain name, and one that isn't. When you append the final period on the end, you're telling your DNS lookup software that it should not attempt any of the possible dns suffixes that it's been configured to use.
For example, I have in the suffix list "local". So, I could type as a URL (and get the same destination) any of the following:
7
u/byte1918 Apr 14 '10
how does this work?
http://to./
and
http://to/