It's a cat-and-mouse game between the pirates and copyright protection. It swings different ways in different years and it's been going on since...oh...Reagan's first term or so.
Right now, for example, I have Battlefield 2. I play it on two different computers. But I don't like to be taking the original CD back and forth. I could copy the CD (using the link above as a good reference), but in my case I took a different route.
First, I made an image file of the Battlefield cd (Clone CD demo works, Alcohol 120% also does it). So now I have a 600MB .iso file on my hard drive (a wee bit of hard drive space is needed). I had to follow settings I found on the internet to copy it correctly...not real hard, check the right box, etc. depending upon which game is being copied.
Next, I installed Daemon Tools (freeware). Daemon Tools emulates a CD-ROM drive on my computer, it even gets a drive letter and shows up that way on your computer. For example, I have drive letter K:
Next, I "mount" the Battlefield .iso image file to my virtual CD-ROM drive, so the computer thinks I have a CD-ROM labeled drive k and the Battlefield CD is in it.
So I don't even bother with the CD at all anymore. There's several ways to do it, this is just one of them.
Notes:
Alcohol 120% will also do the Daemon Tools CD-ROM drive emulation
On some games like my Battlefield 2, I wouldn't be able to play both computers online at the same time because this particular game has an online account "key" that came with my packaging. Some games are like that.