What does Steam put on my PC?

[Quall999]

Ahoy there! For future reference, I am talking about Steam and Boxed titles, not the online purchase thing.

I have been out of the PC gaming loop for some time. I was looking into buying The Orange Box, but want to know about Steam first. I searched this section of the forum, but didn't get too much information other than that it is an online activation.

I search google for little answers, and so far I only know
1. It is required to play the game.
2. The activation is online.

I get mixed answers on the second item there. Assuming that I buy the box and avoid the online purchase, will I need to connect online every time I want to play the game, or does it only need to be activated once and connected only once?

My main concern and the reason for the topic title is that I do not know what exactly is being installed on my computer. Does Steam try to call home every time my computer starts? Does it put some hidden key in my registry that is never removed? I don't want my PC to slow down at boot and I only plan to have Steam running when I play the game, that is possible correct? I realize this is a backup forum, but this is somewhat related to copy protections. I try to keep a clean registry and hidden crap bothers me.