How to spy on another phone?

Nowadays, people can spy on another phone due to the technology called spyware. Its asset is the possibility to record data from another phone and deliver it to its user. Meantime, it runs in a background and remains undetectable for the phone’s owner.
Spyware might be represented in the market as spy application or parental control app. These are TheTruthSpy, copy9, mSpy, Qustodio, Teen Safe, Mobile-tracker.