First, its sound oscillators for low-frequency audio waves. Then Radio wave oscillators varying from low frequency AM to high frequency Cell-phones. Then microwave oscillators in the giga hertz. What is the highest frequency that electronics can generate and what are the uses?
http://spectrum.ieee.org/semiconductors/devices/transceivers-to-conquer-the-terahertz-frontier/2 is a good read to start from.
There isn't a hard upper limit, but it gets increasingly painful to work with and starts crossing over into the domain of optics and optoelectronics. One of the limits mentioned there is that it takes a distinct amount of time for a signal to travel through a transistor, so transistors become unusable; that article claims this is somewhere in the hundreds of GHz.
Parasitic capacitance also becomes an issue; the higher the frequency, the easier it is for the signal to just leak out through the few pF capacitance between your wire and the nearest other bit of metal. This effect limits the usable frequency of signals on breadboards to a few MHz.
According to the Wikipedia article Terahertz radiation, "in this [terahertz] frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques."
This implies that the next lower band, millimeter wave, is the highest where "conventional electronic devices" can be used to produce signals.
Millimeter wave signals have frequencies between 30 and 300 GHz.
Applications include point-to-point wireless communication, imaging (as in airport scanners), and remote sensing.