What is 3G?

I assume you mean 3G in the cellular context.

So, here is the definition from Wikipedia:

"3G (or 3-G) is short for third-generation technology. It is used in the context of mobile phone standards. The services associated with 3G provide the ability to transfer simultaneously both voice data (a telephone call) and non-voice data (such as downloading information, exchanging email, and instant messaging). In marketing 3G services, video telephony has often been used as the killer application for 3G.

3G networks are not upgrades of 2G networks and do not operate on the same frequency spectrum; rather, entirely new networks need to be built and new frequencies need to be assigned to mobile operators. Induced euphoria led to huge spectrum-licensing fees in many countries, especially in Europe, where spectrum auctions went into the billions of euros...."

The wikipedia definition is useful, but you'll find out more here.  The key thing is speed - see the phrase:

"The immediate goal is to raise transmission speeds from 9.5K to 2M bit/sec."

That means that 3g phone services have the bandwidth to give you things like video and TV on your mobile, video calling, access to online gaming, or just faster mail or multi-media messaging. 

3G is the third generation of mobile phone standards which came after 2G. It is based on the (ITU) family of standards.3G technologies enable network operators to offer users a wider range of more advanced services while achieving greater network capacity. The first country introduced 3G on a large commercial scale was Japan, in the year 2001. Now it’s well available across the globe.