what is the Difference between tortoise svn and CollabNetSubversion.....
One is a graphical client (or rather, a shell extension for Windows) and the other is a command-line client1. Usage differs greatly as do convenience features, however, the basic feature set to work with Subversion repositories is exactly the same as they both use the same library.
As for the Subversion server, that's actually a different piece of software, although it is sometimes distributed alongside with the command-line client.
1 Side note: I have long since stopped using the CollabNet-branded version because it's behind a registration wall. I found Slik SVN as a good alternative. It's exactly the same command-line client you get from CollabNet, sans the hassle in downloading it.
One (Tortoise) is a GUI and the other is a command-line tool. However, both are built with the same SVN libraries and can be used together to access the same repositories. Some things, like using within a script, are easier with the command-line (and vice versa, of course) - I find it desirable to have both installed.
Subversion, which is now supported by Apache, is a source control system.
Svn is the abbreviation for Subversion.
Tortoise SVN is a Subversion client, implemented as a Windows shell extension.
You have to install Subversion if you want a source control system.