laymans terms
ssh is a internet protocal for secure ftp and telnet. ftp and telnet are known for secrity problems. ssh is much better. OpenSSH is a open source version of it and very widely used. OS X comes with it built in. When you boot up, you should see it starting up in the window at boot time. I will have to look at home to see how to disable it at boot time in OS X. I think its in the system preffs. In unix, apps that run in the background are called demons. In this case its sshd that is running in the background. If your on a dial up or a frequently changing dinamic IP address, then you probably dont have anything to worie about.
Basicly, if you dont use it and you have a "on-all the time" internet connection, then you should turn it off the be safe or block it at your firewall. If you don't know if you using it or not... your not. There should be a new version of it rolled into what ever the next version of OS X is.
For the other tech talk I was doing... open source software is often distributed as just raw source code. You have to compile the code with a compiler for the particular computer language that it is writin in. Compilers take source code that is easy for humans to understand and write and change it into machine code or binary code (all the ones and zeros) If you installed the developer CD with OSX then you have the gcc C++ compiler all ready installed. Once a new open source app is out for a while, some one will test it and make whats called a package for each UNIX like OS. packages are installers that have all the pre-compiled binary files ready to go for your OS.
if you want to know about compilers, packages, or ssh, let me know... I could go on for hours... and I dont want to bore you if you dont want any more information than that.
-evildead