There are so many names for this e-liquid flavour but originally the name comes from 555, a brand of cigarette manufactured in the United Kingdom by the Ardath Tobacco Company.

You can find many descriptions from e-liquid producers.

But what is the taste like is very personal matter. Some would say it's earthy, nutty taste with a nice hint of tobacco or chocolate and hazelnut with a rich but not to sweet tobacco flavor.

We put together 11 different 555 e-liquids from 11 brands (SteamGunk, Mt Baker, Inawera, Hangsen,....) in this 555 pack. I'm sure you'll find the difference among them and a great pleasure in very personal experience of these 11 flavours.
http://www.smokerainbow.com/555-pack-of-11-samples/

Enjoy!