There are a lot of different types of viruses you can run into. Im only going to tell you about the "Common" ones.
1. Trojan Horse Virus is a non-self-replicating type of malware which appears to perform a desirable function but instead facilitates unauthorized access to the user's computer system. Meaning downloading unsecure games and opening up attachments to an e-mail that your unfamiliar with can cause this virus.
2.Worms Virus A program that makes copies of itself for example, from one disk drive to another, or by copying itself using email or another transport mechanism. Meaning they are in a lot of internet browsing, cellphones, instant messeging, etc.. To prevent this "Worm Virus" from slithering through your computer! Be wary of files that has double extensions such as, txt.vb or jpg.exe. aviod opening those types of file names.