SecuROM is a class of DRM that runs disc-checks and does online activation. It is meant to tie a game to either one disc or a certain number of online activations. The disc check means you need to carry the disc with you wherever you play, but this isn't a big deal since we had been doing this for years before online retail.
The problem comes up with the online activation. The activation method basically gives you a number of tokens. When you install the game on a machine, the games activates itself and uses one of these tokens. When you run out of tokens you can no longer install a new copy of the game. Effectively, this locks you to a specified number of computers.
This seems like a fitting idea, until you realize that people upgrade and buy new computer. They even reinstall their operating system. In all of these cases, you have to use another token to activate your software again. In this way, you can see how people can legitimately run out of tokens and need more. You can also extrapolate that this will get some people mad.
This can be clearly seen with BioShock. BioShock, using SecuROM, came with a limit of five activations and, at first, there was no way to get your activation back. As expected, some people had trouble. And as one user tactfully put it, "2k has screwed us over. We are renting this game."
The developers of BioShock quickly came out with a Activation Revoke Tool to allow recovery of activations, but the damage had been done and the backlash continued and six months later, the activation limit was completely removed. Even more telling, when BioShock 2 came out, activations had been removed and SecuROM was relegated to disc-checking only.
This is a popular trend with companies that once used SecuROM's activation deciding to jump ship. Other's have seen the hassle online activation has caused and decided to avoid it all together.
But with all the problems that SecuROM causes, does it work? The answer is no, it does not. BioShock was cracked within days of its release. Spore, which has SecuROM, was the most pirated game of 2008. So, for all the hassle SecuROM causes customer, it in no way stops hackers from cracking the game. It even makes some people turn to pirating.