If you specify a font family and someone looking at your site does not have it installed, they will not be able to see the typeface that you have specified.
Some ways to ensure that this does not happen:
Google Fonts is a catalog of fonts that is always accessible (to anyone connected to the Internet). They are free and open source, help make the web look better, and they load pretty fast.
You'll notice that many of the Google Fonts have different styles which are specified by numbers ranging from 100 to 900 (closer to 100 is lighter/thinner and closer to 900 is thicker/bolder). To implement these, you will also have to specify that style number using the font-weight property in CSS. Make sure you remember to embed the font style and use the HTML code from Google!
Bolding: You can try and make a Google Font bold by using the font-weight property. You may not notice a significant change to bold because this font does not have a default bold. In this case you will have to set the font-weight using the number (bold is usually 600 or 700).
font-weight:bold; /* if the above line does not make your font look bold try this. But make sure you have selected the style and embedded it properly! */ font-weight:700;
Italicizing: If you wanted to use one of the italic styles available, you will also need to use the font-style property. For example for Roboto Thin 100 Italic, use:
font-family: 'Roboto', sans-serif; font-weight:100; font-style:italic;