Security
The internet was not originally designed with security in mind. This means that current methods have been patched onto the existing layers of the internet.
Denial of Service
This method of network abuse uses several computers to send traffic to a server in order to overwhelm it.
Packet Sniffing
On broadcast media, like Ethernet and WiFi, you are able to record the transmitted packets for later analysis.
These packets include all transmitted data so sensitive information, like cookies and passwords, can be captured on encrypted sites.
IP Spoofing
Packets can be sent with a false source address. This can complete actions on behalf of an already authenticated machine.