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→Boosts
Boosts generally do exactly what I describe above: emphasize a peak frequency range to focus the distortion. They are often used for metal to "tighten up" an amp by boosting the upper-midrange and treble frequencies, while often dialing back the muddier bass frequencies.
Additionally, however, boosts can add their own tone, adding compression or , touches of distortion, certain resonances , or other weird stuff. Most players will tell you there's a bit of magic or mojo in their preferred boosts. Personally, I usually prefer straight up EQ and would prefer to change the amp (model) for the mojo. I find some boosts can "wash out" the signal a bit. But of course it depends on your preference and the actual pedal/device being used. The only way to really tell is to experiment. Boost pedals often use solid-state clipping that behaves differently from typical tube distortion. It often reacts more quickly, allowing peaks during the pick attack to distort with little compression but without distorting the sustain of the note(s). The distortion is hard-clipping, that can sound buzzy or harsh, adding lots of harmonics all the way to very high frequencies. Some feature '''asymmetrical clipping''' which emphasizes certain harmonics in different ways than tube distortion. While these kinds of solid-state clipping may sound terrible on its own, running it into a distorting tube amp can tame the harshness and create a pleasing sound, where the attack on notes gets more emphasis, almost becoming quite noisy or "swishy". This is particularly evident on palm mutes, giving them a "djent" sound rather than a percussive one, while their sustain produces a "chunk" as the tube distortion compresses the sustained low frequency sound.
The standard for boosts is the Ibanez ''Tube Screamer''; however, due to the variety of both the number of ''"Tube Screamer"'' product lines and variances in components used over the years, I think a more accurate standard is the Maxon ''OD808''. Maxon actually produced the original ''Tube Screamer'' for Ibanez and claims its ''OD808'' is true to the original circuit. Another that seems to get lots of praise is the MXR ''Zakk Wylde Overdrive'', as well as the Maxon ''OD820''. While I haven't used these myself, I have lots of awesome profiles on my Kemper where they were part of the profiled signal chain.