What exactly anti virus is? what are they made up of?

Antivirus software is software that attempts to combat malware, such as computer viruses, computer worms, rootkits, and Trojan horses.

Antivirus software typically employs a variety of strategies, including searching for known patterns in malware executable code (signatures), and emulating a program in a sandbox to see if it acts maliciously. Success depends on striking a balance between false positives and false negatives. False positives can be as destructive as false negatives.

Antivirus software has disadvantages to the user as well. It often causes a significant decline in computer performance, it may present computer users with a decision the user may not understand. Antivirus software generally works at the highly trusted kernel level of the operating system, creating a potential avenue of attack.
There are two common methods that an anti-virus software application uses to detect viruses. The first, and by far the most common method of virus detection is using a list of virus signature definitions. The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is to use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms’ have yet to create a signature for.
Many users install anti-virus software that can detect and eliminate known viruses after the computer downloads or runs the executable. They work by examining the content heuristics of the computer's memory (its RAM, and boot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives), and comparing those files against a database of known virus "signatures". Some anti-virus programs are able to scan opened files in addition to sent and received emails 'on the fly' in a similar manner. This practice is known as "on-access scanning." Anti-virus software does not change the underlying capability of host software to transmit viruses. Users must update their software regularly to patch security holes. Anti-virus software also needs to be regularly updated in order to gain knowledge about the latest threats.
One may also prevent the damage done by viruses by making regular backups of data (and the Operating Systems) on different media, that are either kept unconnected to the system (most of the time), read-only or not accessible for other reasons, such as using different file systems. This way, if data is lost through a virus, one can start again using the backup (which should preferably be recent). If a backup session on optical media like CD and DVD is closed, it becomes read-only and can no longer be affected by a virus. Likewise, an Operating System on a bootable can be used to start the computer if the installed Operating Systems become unusable. Another method is to use different Operating Systems on different file systems. A virus is not likely to affect both. Data backups can also be put on different file systems. For example, Linux requires specific software to write to NTFS partitions, so if one does not install such software and uses a separate installation of MS Windows to make the backups on an NTFS partition (and preferably only for that reason), the backup should remain safe from any Linux viruses. Likewise, MS Windows can not read file systems like ext3, so if one normally uses MS Windows, the backups can be made on an ext3 partition using a Linux installation.

Virus are basically programs which are programmed to do some operation which is not desirable for the user.
Antivirus does the opposite,they monitor the programs doing those work and force them to stop doing the same.
Antivirus basically have more more processor power so that they can kill the virus which have less processor power.
this way they work.