A virus is an unexpected piece of code that embeds itself into a file or document and spreads from one file to another usually to do mischief or serious harm. Boot sector viruses are most commonly activated when an infected floppy disk is left in the drive and the system is rebooted. This sector is often the first place your system reads from when booting up from the hard drive. Then, when the computer boots, the virus will be loaded into the system's memory. Once the infected program is activated, the virus is placed into your system's memory and may duplicate itself further.
Macro viruses are currently the most commonly found viruses.
They infect files run by application that use macro languages like Microsoft Word or Excel. The virus looks like a macro in the file and when the file is opened, the virus can execute commands understood by the applications macro language. Multipart viruses have characteristics of both boot sector viruses and file viruses. They may start out n the boot sector and spread to application, or vice versa. There are also different terms for threats - depending on how they work.