A rootkit gives a hacker or remote user access to all of the folders on your computer, both data and system folders and files. While it is not directly destructive to your computer like a virus or designed to spread like a worm, rootkits are used to gain control of your desktop by hiding deep within your computer which can be far more devastating. They can infect the BIOS, which is an important part of your system that is independent from the operating system you are running which can make them very difficult to detect and remove. Rootkits also run at the same permission levels as a system administrator and antivirus programs which can cause additional problems with detection and removal as the computer cannot tell which program has the greatest authority.
You can become infected by a rootkit by downloading and installing software and programs, sharing disks and USB drives and infected web pages.