Web Safe Fonts
Web safe fonts are those that are available on all computers so that your fonts will remain the same universally. There very few of them as can be seen in the screenshot to the left.
One font this isn't really safe is Impact. This font is not web safe on iOS devices like the iPad and iPhone.
In general, fonts should be kept constant throughout a site using variations in size, color and typeface to draw attention to important text.
Most fonts are available in Regular, Italic, Bold and Bold Italic and that should be enough for most situations.
If you do decide to use non-web safe fonts, they will work if your visitor is on a Mac but the chances are that Windows users will see something completely different.
When a browser loads a substitute font your layout will be effected and this can turn your web page design into a mess - be warned!