What is the difference between a mixture and a composed substance?
From the definitions in your textbook, it seems that a mixture is a combination of two or more substances that--though combined--maintain their chemical identifiability. Examples of mixtures might be:


Water/Alcohol Mixtures
Salt/Pepper Mixture
In pharmacy, pills, pastes/creams, and suppositories are mixtures


(there are many example of mixtures)

The consituent compounds might be separated with somewhat mild techniques, including dissolving in certain solvents, distillation, and crystallization.

Composed Substances (according to your textbook) are mixtures of two or more species that under go some chemical reaction. The species involved could lose most of their identifiability, becoming completely different compound(s).  Loosely speaking, chemical reactions can not be reversed by simple separation techniques.

(Of course, there is some arbitrariness here: particularly in what comprises "most" of their identifiability and what defines "simple" separation techniques.  Also, mixtures are actually held together with bonds, just like in composed substances.  They may be weaker, but even "weakness" of the interaction is a subjective concept.)