“Internet addiction is described as an impulse control disorder which does not involve use of an intoxicating drug and is very similar to pathological gambling”. (source: Net Addiction).
In plain terms it means getting addicted to internet.
According to this report, Internet addiction can cause depression
You can find the signs and symptoms of IAD (Internet Addiction Disorder) over here
And to find out if you are suffering from IAD, you can take this test