You can use STEAM on any PC you choose.
The games are tied to your account, not a PC.
The only exception are a few games that require 3rd party activation. For example a SecROM game. It may only allow a limited amount of activations at any given time.
The Store Page will indicate if it has any 3rd party DRM though, and the details.
If you use Steamguard though, youll need to supply not only your account name and PW, but a unlock code which is emailed to you, for the other PC to access your account.
This is a good thing IMHO, and would suggest using it.
Also, bear in mind only 1 PC may be online at a time.
So you can NOT, for example, have Steam in Online mode on both PCs, and play a Co-Op of Left4Dead 2.
Youll need 2 seperate accounts and 2 copies of the game for something like that.