I want to know what are the risks of using the WiFi internet instead of the cable.
why the cable is more secure ? why the the WiFi is more vulnurable ?
The main difference in the exposure of your network traffic. On a wired network, there is a physical cable connecting your computer to your router to the Internet. The only way to listen in on the conversation is to steal the cable.
On a wireless network, your computer shouts to your router over a radio frequency; any other devices within the range of your transmitter can read your signal as well. To make it harder to read the exact content, wireless networks usually have some sort of encryption (e.g. WEP, WPA, etc). While this makes it more difficult to read your data, it is still possible to listen to it, save it, and brute force crack it to determine the contents.
On guided media (ie: wires) you can sniff packets only by attaching yourself to the cable. This isn't that easy in most cases.
Wireless signal can be instead heard by anyone. For this reason Wifi connection usually are encrypted, while wired connections are not.
The security of your network is not determined by wireless or wired. Its your router / network device you are using and the type of security you use. Wired is the most efficient way of connection with better and uninterrupted speeds as where wireless has limited speeds and range