1. There are six main types of reactions
Synthesis
Decomposition
Combustion
Single Replacement (Single Displacement)
Double Replacement (Double Replacement)
Neutralization

2. Synthesis
A combination of two or more substances to form a larger substance.
A + B  AB
Examples:
Hydrogen + Oxygen  Water
Sodium + Chlorine  Sodium Chloride

3. Decomposition
The breaking apart of one substance into two or more parts.
AB  A + B
Examples:
Water  Hydrogen + Oxygen
Calcium carbonate  calcium oxide + carbon dioxide

4. Combustion
A special form of decomposition which involves a reaction between a hydrocarbon (has C and H) and oxygen to form carbon dioxide and water vapor.
Example:
Methane + oxygen  carbon dioxide + water

5. Single Replacement or Single Displacement
An element in a compound is replaced by a free element.
A + BC  AC + B
Examples:
Potassium + water  potassium hydroxide + hydrogen
Lead (II) sulfide + iron  iron (II) sulfide + lead

6. Double Replacement or Double Displacement
Two elements switch places with each other.
AB + CD  AD + CB
Examples:
Zinc chloride + sodium bromide  zinc bromide + sodium chloride
copper (II) sulfate + silver nitrate  silver sulfate + copper (II) nitrate

7. Neutralization
A special form of double replacement in which an acid and a base form salt and water.
Examples:
Hydrochloric acid + sodium hydroxide  sodium chloride + water
phosphoric acid + calcium hydroxide  calcium phosphate + water