The RF BJT is a bipolar junction transistor that is designed to operate efficiently at higher RF frequencies in the 100s of MHz into the GHz range.  A standard BJT would have unwanted losses in power transfer and may have bad characteristic transistor functionality at those same frequencies.

RF BJT components are used very frequently in power amplification applications.  For high frequency signals, like RF, they are designed to have a very good linear response, meaning that the proportionality between input signal and output signal remains quite stable as long as the linear region ratings are followed and maintained.  They can also be used as a switching transistor like a MOSFET, operation outside of the linear region and within its saturation mode.  In this case, the signal must be translated into a pulse-width modulation and be passed and amplified through the BJT, where the signal is changed back to the original signal.

RF BJTs typically offer some superior characteristics as compared to RF MOSFETs.  They maintain a higher linearity in their linear region and are much cheaper for the same functionality. They have much lower output impedances, and have much higher gain potential.