There are a very limited number of fonts that are "web safe."
When a web page (or formatted email) contains text, it is the visitor’s computer that is controlling the display. It is being told “display the following text in the following font.” So it depends on the font being installed on the computer on which the visitor is trying to view the website or email.
The alternative is to display text in the form of a graphic (image), which provides exact control. There are liabilities to that which dictate limited use. It adds a lot of work. For web pages it adversely affects SEO (Search Engine Optimization). And for emails it means the text in question won’t be visible until or unless someone chooses to download pictures.
We use a variety of fonts in headlines, banners and the like to focus attention, fit the clients' logo, match the flavor of their business etc. The bulk of the text will then be in one or two of the few web safe fonts.
Since not all computers have the same fonts installed, we often specify a series of font choices, so that in most cases the copy will be displayed close to as intended. For example “Arial, Helvetica, san-serif” meaning use Arial if installed on the computer, if not, Helvetica, if that fails, use any san-serif font you have. Arial and Helvetica are very similar fonts, but PCs have Arial and Mac’s Helvetica.
So text won't look exactly the same on a Mac as on a PC.
The fact that we list a font doesn’t mean we recommend it. Comic Sans MS is way overused and recommended only in very special cases. Impact is only for headlines and in certain cases. Courier is a typewriter font, only for use if you are trying to look like typing.
The first group are almost always safe. Verdana for example should work unless they are running Unix or it's an ancient computer (small percentages).
Windows fonts / Mac fonts
Arial / Helvetica
Comic San MS
Courier New / Courier
Times New Roman / Times
The second group are less certain to be on a computer.
Arial Black, Gadget
Lucida Sans, Lucida Grande
This last group are even less certain of being available on any particular computer trying to display the web page or email. Again, these are always backed up with a font in the first list.