Spy is related to an individual finding information which is termedas secret and
confidential without taking permission of holder who is responsible for holding the information. The spy is also known as espionage. Espionage is termed as clandestine which act as the main holder of the information . The clandestine may change its view regarding the information once it comes to know that the information is in wrong hands. One can know more about spy or espionage or clandestine by reading lots of articles or visiting to sites.
general meaning of spy is secret agent . And througout history incidents of spy has been recorded. One can give a look in our ancient writings also when chinese and indian armed forces were using Sun-Tzu and Chanakya who were collecting information on sources of deception and subversion. Even the founder of maurya empire chandragupta maurya who was the student of chanakya was using the spy sources such as assassinations, spies, and secret agents as mentioned in chanakya's arthasastra. Even spy incidents one can see in modern espinages also.
Lots of countries are using spy as source of providing security to one's countrymen by applying it in defence areas. In each country various departments play an important role in gathering spy information. For this they use many intelligence collection techniques overlead other countries defencely. One can understand the concept of spy by following the example of former USSR which used human sources instead of research in open sources for doing spy and due to lackness in the country's defence spy that the country shattered. In the same way united states too had spy methods and it focuses on technological methods like SIGINT and IMINT.
In short one can say that in today's time spy term can be used in a meaningful way rather than taking only negative aspect of the word. Not only in defence spy can act as a source of getting used but also in lots of ways such as in technical fields, scientific and research fields it can play an important role.