The substances (in the molecular or ionic form) which are capable of oxidizing other substances by accepting electrons from them are called oxidizing agents. Thus, all chemical substances which can accept electrons from other are oxidizing agents.
Strength of an oxidizing agent depends upon the electro negativity of the element. More electronegative elements are stronger oxidizing agents. The substances which can bring about reduction of other substances by losing electrons to them are termed are reducing agents. Thus, all chemical substances which can lose electrons easily are reducing agents.