It's not something you hear about everyday, but it's more common than you think. Consider your computer has been compromised, do you know about it? Is there an attacker logged onto your PC right this moment? Chances are - you wouldn't know. That's just what a rootkit is. It's a tool that cloaks malicious activity on your PC while giving an attacker full administrative access. It's not something you'd like to have.
I've been studying more and more about rootkits and it's something I'd like the general population to be aware of. Rootkits were utilized by many corporations, mostly for good reason, but it's not always good to have a back door open on your PC 24/7. Consider Sony BMG Music, in 2005 they've installed rootkits on all their CDs to ensure users wouldn't access areas of the CD that they didn't want the general audience to see. It's because of this that almost everyone's computers utilizing Sony's CD with the rootkits were compromised, and a class action lawsuit was dealt to them. The cause was an attacker found how Sony was cloaking it's CDs and it decided to take advantage of the situation. This is possibly one of the biggest marks on Sony's reputation today, second to the server hacking in their popular game Final Fantasy Online.
Rootkits were introduced as a positive concept, to allow authorized access to another PC. Nowadays rootkits are considered malware and most companies won't even touch them. There're ways around needing to use a rootkit for good, but some methods you just have to install a rootkit. For example, deleting data on a stolen computer remotely. Even then there should be an alternative to a rootkit granted the risks involved.
I suppose my only reason for bringing this up is to raise awareness of rootkits. I've never had one, but they're very easy to get. If you have a rootkit, don't expect removal to be easy. You'll most often need a full OS wipe. In some cases, it could be a hardware rootkit, which you'll need to replace the entire part, maybe your entire PC.
Be safe, protect yourself from rootkits. Here're some valuable resources you can use to learn more about what rootkits are, and where you can get proactive defense to prevent getting one. This list is not exhaustive, but serves as a good start.