Watch the Boost online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide
The simplest effect pedal is the clean boost. All it essentially does is boost the signal of your guitar. Guitar players loved this because it allows us to hit the front end of our amp with more signal. This increased volume level will drive the input section of your amp harder and provide some more gain. The earliest boost pedals used a germanium transistor and was often in the form of a treble booster. The most famous treble booster is the Dallas Rangemaster which is rumored to have been used by Eric Clapton on the Bluesbreakers record, often referred to as the Beano record or Beano tone. (This is because Clapton is reading a Beano comic on the record cover). At the time Eric was using a Marshall JTM45 2x12 combo (commonly referred to as the Bluesbreaker because of this recording) and a Les Paul and man does it sound good. Allegedly he was using the Rangemaster to push the front end of the amp into more distortion. This has never been confirmed to my knowledge but is the source of much Internet speculation. But I have played an old Bluesbreaker amp with a Les Paul and it sounded pretty darn close. Both Brian May from Queen and Ritchie Blackmore did use treble boosters for sure in their rigs to get more gain out of their amps. Germanium transistors are very inconsistent and are subject to temperature changes so they can be finicky. They also certainly color your tone to a large extent, which is something you may want. Clean boosts that use silicon transistors are much more common and reliable, they can also boost your signal without effecting your tone too much. For a general clean boost I would go for a modern one. If you want an old school sound, I'd check out a germanium based unit. Or like me, I'd get both.