Rufus is a simple utility which makes it easy to build bootable USB drives from ISO images.
Of course if you have an ISO image then you could simply burn this to a CD or DVD. This won't help you much if your target system doesn't have an optical drive, though. USB keys are also easier to carry around, less likely to be damaged, and faster, which may be important if you need to use the disc often.
Rufus doesn't require any installation, and is generally very straightforward to set up. It'll automatically detect your USB drive, for instance, so you may not have to select that. Getting started could be as simple as browsing to your ISO image and clicking the "Start" button: the program will then walk you through everything else it needs to know.
If you need more control, though, you're able to configure how the USB drive will be formatted (file system, cluster size and so on). There's now support for both UEFI and GPT boots. And the program even includes some advanced format options which may help your drive be recognised on older PCs, although normally these shouldn't be necessary.
And if this is a task you need to carry out often, then you'll be pleased to hear that Rufus is reasonably speedy. We created a bootable USB version of the Windows 8 Professional image, for instance, and found the program was around 25% faster than another popular boot drive creator, UNetbootin.
Note that this is the portable version.