on flat bar bikes generally Revoshift is a collar set in from the end of the handlebar, you twist it to change gear, the collar will have numbers on it corresponding to the rear cogs or front chainring, depending on which side of the bar they are mounted.
Or you can get ezi-fire levers that you change by flicking your finger or thumb, numbers appear in little windows mounted on the gear assembly
Revo shift are usually SRAM and ezi-fire Shimano, but I believe that shimano also make a revoshift style gear change.
On drop bar bikes there are three main manufacturers of gear changers.
Shimano -(Japanese fishing tackle manufacturer) their system uses an integrated brake gear shift lever called STI's
shimano (something )intergrated system.
SRAM - (Johnny come lately American upstart) rival/red, works on a similar principle and has a curious double tap shifting action.
and Campagnolo (long Established Italian engineering company, who hand make all their bike components, and mill parts for Ferrari cars)- Ergo's which have a gear paddle located behind the brake lever for changing down, and a button located on the side of the hood or housing for changing up.And are an absolute dream to use.
there is only one choice for the serious cyclist from these three, which is Campagnolo <lights blue touch paper and retires> LOL!!
