"Parallels" is virtualisation software for the MAC, allowing you to create one or more Virtual Machines within OSX that can run a guest OS such as Windows or Linux. OSX is running with the guest OS in a "window", if you will.
It is quite different to a dual-boot system, where either - but not both - of the installed OS can be running at any one time.
AFAIK, Boot Camp is free, Parallels is not (VMWare is also not free, but Sun's VM is free).
SD