"Single speed" refers to how many gear ratios a bicycle has, while "fixed gear" refers to how the gear is attached to the hub. "Single speed" means that a bicycle has only one gear. "Fixed gear" means that the gear is fixed to the hub without a freewheel, so if the rear wheel is turning, the cranks are also turning.
Many single speed bicycles use a freewheel to allow coasting, and are therefore not fixed. And it's actually possible for a fixed-gear bicycle to be multi-speed, able to shift between multiple gear ratios using something like a Sturmey-Archer S3X hub, although this is very uncommon; nearly all fixed-gear bikes are also single-speed.

Many single-speed bikes come with flip-flop hubs that can mount a fixed cog on one side and a freewheeling cog on the other, so your bike purchase isn't committing you to one particular scheme.

Sort of. Since single-speed road bikes do not require shifters, derailleurs, or multiple sprockets front and/or rear, they can often manage better build quality at a given price point than derailleur-equipped road bikes do. I wouldn't say it's an enormous gain, though.


It really depends on the use case. While pedaling at a given speed and cadence, a single-speed isn't giving anything up to a multi-speed bike. But if your riding involves highly variable conditions or gradients, or especially in a spirited group ride which demands highly variable efforts from a rider, they can be at a significant disadvantage.