Do I need anti-virus software?
If you are used to Windows or Mac OS, you are probably also used to having anti-virus software running all of the time. Anti-virus software runs in the background, constantly checking for computer viruses that might find their way onto your computer and cause problems.
Anti-virus software does exist for Linux, but you probably don't need to use it. Viruses that affect Linux are still very rare. Some argue that this is because Linux is not as widely used as other operating systems, so no-one writes viruses for it. Others argue that Linux is intrinsically more secure, and security problems that viruses could make use of are fixed very quickly.
Whatever the reason, Linux viruses are so rare that you don't really need to worry about them at the moment.
If you want to be extra-safe, or if you want to check for viruses in files that you are passing between yourself and people using Windows and Mac OS, you can still install anti-virus software. Check in the Ubuntu Software Center where a number of applications are available.