How to combine computers?

[Joseph Smidt]

I'm sure you all have seen places where their are many computers networked together.  Form any one of them you can access any of the accounts of the others.  Multiple computers running the same system, like at office computers all tied together.  My question is how do you do it?  I have two computers and I want it so I can be one either one and run the same Debian accounts.  I have hardware to link the two computers, but how do I connect them software wise?