Read section 9.10
In this type of reaction a single more complex substance is broken down into 2 or mroe compounds or elements. Most of these such reactions are endothermic, because energy is supplied and used in the reaction. Also, it is of note that decomposition reactions are the opposite of Direct Combination reactions.
Compound -> Element or Compound + Element or Compound
AB -> A + B
Examples:
MgO(s)
-> Mg(s) + O2(g)
2NaCl(l)
-> 2Na(l) + Cl2(g)