What is Inside?

There's a lot of information that needs to be processed when it comes to handling of a fire. The type of fire is the first step (A, B, C, or D fires) and then the appropriate steps may be taken to utilize the proper fire extinguisher. The type is determined by what's literally inside and there are a few to distinguish from so all of the types along with a chart are available below.

WATER EXTINGUISHERS
These extinguishers are only good on ordinary (Type A) combustible materials such as wood, paper, etc. It would definitely not be effective on electrical fires (Type C) due to the risk of an electric shock and it would not prove useful on liquid fires (Type B) because you run the risk of spreading the fire. There are water mist extinguishers available that can work on Type C fires and on Type A fires that run the risk of Type C fires.

CO2 EXTINGUISHERS
CO2 Extinguishers work best on Type B and C fires; the sheer force caused by the extinguisher would blow a burning Type A fire around whatever room it is contained in, spreading the fire in the process. The CO2 ext. works by displacing the oxygen at the surface of a fire (oxygen is the fuel of every fire), killing the fire slowly but not extinguishing completely until the source has been removed.    

DRY CHEMICAL EXTINGUISHERS
Dry chemical ext. are effective on more than one type of fire since chemicals are usually compatible with most fires. Dry chemical ext. that contain sodium or potassium bicarbonate are best suited for Type B and C fires or a combination; extinguishers with ammonium phosphate are suited for all three types or a combination.

Below is a chart from the University of Alberta's Department of Earth & Atmospheric Sciences' website summarizing all the points above in a poignant chart.