Just a couple of examples of serif and sans-serif font family specifications.
font-family: Arial, Helvetica, sans-serif; font-family: Verdana, Arial, Helvetica, sans-serif; font-family: Geneva, Arial, Helvetica, sans-serif font-family: "Times New Roman", Times, serif; font-family: Georgia, "Times New Roman", Times, serif; font-family: "Courier New", Courier, monospace;
THe following fonts are considered web-safe (meaning they should be present on any system):