What is Runet?

Runet is basically Russian Internet.  Or, to be more verbose, it is the network of sites, publishing context in Russian language.  While some consider only the web sites in the top level domain .ru as the true Runet, it is more acceptable to view other sites as part of it, as long as they are in Russian.  This, for example, includes a huge number of Russian-speaking members of the one and the only LiveJournal, Russian sections of such news channels as BBC and Deutsche Welle and so on.  There are a few other Russian-oriented web sites outside .ru, which should be included.

Runet is quite large.  There are more than 200 million people in the world, who have Russian as their mother tongue.  They live mostly in the Russian Federation, but large communities can be found in all exUSSR countries, in US, Germany and other European countries, in Israel and Australia.  Not only native Russians know the language, considerable portions of the local population in former Soviet Union republics and eastern Europe learned it in schools and quite a few of them are bilingual.

Is it something unique, Russian-only?

I suspect not.  Given the numbers of people, who can speak Chinese, Spanish, Arabic and a few other popular languages and who have access to the Internet, I wouldn't be surprised if there were chinanets, arabnets and so on.   The Internet is large - there is place for everyone. If you know more, please tell me via comments.

Is it good for me?

It depends, really. If you have (or want to have) any relationship with Russia or Russian speakers, than Runet would be of an interest to you. Otherwise its existence shouldn't really disturb you or your day to day life.

Vrunet?

Here comes an interesting twist.  In Russian "vru" means "I'm lying".  Since Russians have lost any trust to media after the collapse of the Soviet Union, it is not uncommon to give mock names to popular on-line  papers and news web sites, like Lenta.vru (lenta.ru - is one of first and of the most popular ones), or Vesti.vru (vesti=news).  One should understand, that Russians usually don't take seriously any information found on the Internet.