This kinda sounds like homework.
Anyway, as you seem to have been living under a rock... most residential, and even commercial networks are vulnerable to attacks, most people buy a wireless router/access point, plug it in.. and assume that's all they had to do.
Many networks are unencrypted and have no sort of authentication system... but users aren't entirely at fault, the available encryption techniques are mostly insecure..
WEP, (Wired Equivalent Privacy) was cracked.. years ago, I believe it takes only seconds to find someones key these days.
WPA/WPA2, is a little better.. but I know very little about it...
I'm not an expert, I'm sure there are more qualified people here... up until recently I had no wireless equipment, I've setup a few AP's for fun.. with OpenBSD+authpf.
All unauthenticated traffic was.. served a bogus error page.