There are three types of boosters.
The first are signal boosters. These give a gain boost to the signal running through it and appear to make the guitar louder. Transistor booster preamp stages are often billed as “linear,” “clean,” or “distortion-free” boosters, and are intended to boost the guitar signal before it reaches the amplifier, without adding any artifacts like fuzz or distortion. The “linear” bandied about in the names or literature of many means, in basic terms, that the guitar’s frequency range—and therefore its tone—should be unchanged as it passes through them, only the signal level should be increased.
The second are frequency boosters. These are similar to the signal boosters but instead of boosting the whole signal, they boost one specific frequency range.
The third are harmonic boosters. These boosters are deliberately designed to enhance certain frequencies. Vox offered a Treble Booster and Treble/Bass Booster way back in the mid-1960s, and Electro-Harmonix followed its LPB-1 with the Screaming Bird and more powerful Screaming Tree treble boosters. Though these certainly emphasized high frequencies, as desired by countless guitarists at the time trying to cut through the mix as bands got louder and louder (and often, as a result, muddier), they also offered a general signal boost that had a similar amp-overdriving effect on an amp as the more linear boosters.