Wikipedia says the following:
An amateur (French amateur "lover of", from Old French and ultimately from Latin amatorem nom. amator, "lover") is generally considered a person attached to a particular pursuit, study, or science, without pay and often without formal training. Amateurism can be seen in both a negative and positive light. Since amateurs often do not have formal training, some amateur work may be sub-par. For example, amateur athletes in sports such as basketball, baseball or football are regarded as having a lower level of ability than professional athletes. On the other hand, an amateur may be in a position to approach a subject with an open mind (as a result of the lack of formal training) and in a financially disinterested manner.
Many amateurs make valuable contributions in the field of computer programming through the open source movement. Amatuer dramatics is the performance of plays or musical theatre, often to high standards and often not, but lacking the budgets of professional West End or Broadway performances. Astromony, history, linguistics, and the natural sciences are among the myriad fields that have benefited from the activities of amateurs.