Virus: This is a special type of program that is able to infect itself into a computer without a users permission but requires a users intervention to spread. A Virus is a program that attaches itself to executable files and will cause an infection when a user runs the application that it was attached to. Viruses range in complexity and scope to any of the following: some that just are minor annoyances to those that are so powerful to cause physical damage to computer hardware.
Infect: A term used by computer professionals to indicate that a file has installed itself into a computer,network or system without a users knowledge. A common occurrence of this is when a user downloads a program and it forces the system to shutdown, then carries out its installation instruction before the system is finished booting.
Trojan: A special type of program which may appear to be functioning quite normally; however it is really acting maliciously behind the scenes. This type of program will often get installed when you surf bad websites and our known to install back-doors into your computer and or network. Once a Trojan is installed; your computer will allow the person that sent it to you, to illegally get into your computer. A Trojan doesn't make copies of itself but it does do damage; because it opens up your computer to be easily accessed, and taken over which people call remote access Trojans.
Worm: A worm on the other hand is a type of virus however; the way it infects is much different as its doesn't require user action. For example one worm might send a copy of itself to everyone in your e-mail address book and then turn around and send itself to every one's book from each separate computer. Worms have a unique property which is that they can easily copy themselves to files,folders,computer and other networks in your company, organization and eventually the entire Internet.
Scanner: A program designed to seek out viruses, Trojans, worms, identify them and then remove them. It is also important to know that there are specific files that will target certain types; especially when they are free. Many times people don't have a virus scanner, or have one but it won't remove anything because its just a scannerer and not a remover.. Most scanners are removers; however not all of them are and you can see exactly what a program will do by reading the help file and or the welcome screen when you execute the program.