Technomancers started to appear in 2064, after the Second Crash . At first,people didn’t notice much, but there is a rising paranoia, especially after an incident at a MCT facility in Hong Kong. Here, technomancers were blamed on the explosion that destroyed the facilty and for the brutal murder of at least one doctor. The fact that the technomancers had been subject to experiments was nowhere as widely mentioned.
Since technomancers are able to hack without access to a commlink, they are seen as a threat. Often, their abilities are compared to that of magicians and there are parallels. If a technomancer is kept away from any Matrix access, it has a noticeable effect on his health after a while. They rely on something called Resonance for their abilities. Opinions differ as to what the Resonance actually is. The magic part of the Matrix is as good a description as any for now. Technomancers are able to create sprites and may be able to access the Resonance Realms, a deep and hidden part of the Matrix presumably created by the Resonance itself.
Resonance has a counterpart, Dissonance - corrupted data, basically. Technomancers and sprites can become dissonant and out of synch with the Resonance. The fight against Dissonance is seen as a duty by many technomancers.