Programs or pieces of code that "infect" one or more of the programs on your PC. Basically, your programs "get sick" and start performing in weird ways, which sometimes can lead to a system crash.
Malicious programs that propagate over a network, reproducing as they go. Worms cause the same effects as viruses but they are more dangerous since they spread by themselves.
Hackers scan the open ports on your PC to figure out if they are open or exist at all. If your computer reports an open port, a hacker can send a worm and virus to it. They can even use an open port to take control of your PC.
DENIAL OF SERVICE (DOS) ATTACKS:
This kind of attack happens when a hacker finds a responding port on your PC and sends a huge amount of data to it. The port is just unable to accept all of the data, the system resources exhaust, and the system crashes and denies service.