Here, we will look at how expert journalists have defined and explained citizen journalism:

Who is a citizen journalist?

Anyone who cares to report about what one sees happening. In words of journalism Professor Jay Rosen, the citizen journalists are "...the people formerly known as the audience"

Five basic principles of Citizen Journalism
In 2007, Dan Gillmor, one of the foremost proponents of citizen journalism and JD Lasica put together five basic Principles of Citizen Journalism. These five things are:

accuracy, thoroughness, transparency, fairness, independence.

In short, the citizen journalist must be as dedicated to his craft as a top-notch journalist is.

How many types of Citizen Journalism are there?
A couple of years ago, when citizen journalism was literally on its way up, Steve Outing write a detailed guide to 11 different flavors of citizen journalism. Read this list and then in detail after the jump, in the knowledge that social networking tools such as http://twitter.com are providing new dimensions of reporting and insemination of information. Also keep in mind that Steve wrote this from the point of view of newspapers and how they are opening up to ideas and influences from the people at large. Useful for journalists from a traditional media background.

1. Opening up to public comment
2. The citizen add-on reporter
3. Open-source reporting: Distribute an article project among readers.
4. The citizen bloghouse: This is where most new innovation is happening, on blogs run by people in the community in places around the world.
6. The stand-alone citizen-journalism site: A mix of citizen journalism plus editors who shift, edit and choose stories from readers.
7. The stand-alone citizen-journalism site: Unedited version (no editors, unfiltered voice)
8. Add a print edition: e.g. theprintedblog.com in Chicago [note: this is my addition]
9. The hybrid: Pro + citizen journalism: e.g. newassignment.net, assignment zero [both started by Jay Rosen]
10. Integrating citizen and pro journalism under one roof: e.g. ohmynews.com [Korea]
11. Wiki journalism: Where the readers are editors [e.g. wikinews.org]

9 Alternative names for Citizen Journalism

Mark Glaser at Mediashift has written in detail about these:

Grassroots journalism
Networked journalism
Open source journalism
Citizen media
Participatory journalism
Hyperlocal journalism
Bottom-up journalism
Stand-alone journalism
Distributed journalism

Richard Sambrook's 4 types of Citizen Journalism
Richard Sambrook of the BBC lists out 4 basic type of journalism done by citizen reporters, including how news businesses use their services. Basically, the idea is that almost every major news organization is open to contributions from online citizen reporters, although it is not clear how many of them pay for the content. For that, you must find out for yourself:

1. The use of eyewitness accounts, pictures and video.
2. The integration of user comment or blogs into news coverage.
3. News broken on the web. Sambrook calls this 'real citizen journalism'. Add to this the current trend of people breaking news using their mobile phones on short-text social networking services such as Twitter.com, most famously during the Mumbai Terror blast in 2008 and a couple of plane crashes in the United States when passengers 'tweeted' about the crashes as they were happening. Fortunately for them, they lived to tell the tale in detail afterwards.
4. Using the public to develop and inform our journalism solution.