There are many different types of attacks that could be exucuted on your computer. A hacker could attempt to get your password or IP adress so they can gain acces to your system and attack it. To prevent this you should keep your IP address very secure and tell no-one your password. They could also Snop, easdrop or intercept your information. They can browse your files, they could use a sniffer program to listen to the traffic of a network or they can just intercept outgoing and ingoing emails so the person never recieves them.