Electro is a term used to describe specific styles of electronic music.
It was originally used to describe a subgenre of hip-hop that gained popularity in the early 1980's in Germany, England, and the US. Electro was directly influenced by Kraftwerk and Funk, whereas earlier hip-hop was closer to disco. Defining characteristics of this genre were dominating synthesizer lines and rhythm sections comprised of drum machine beats, rather than samples. Vocals were delivered in a robotic, dead pan matter.
As time progressed, many subgenres grew out of electro, as well as a certain fashion sense and attitude. Foremost among these were electroclash (a hybrid of electro, new wave, and Italo disco) and electropop (a form of synthpop characterized by a cold, robotic sound). It is heavily debated among music scholars whether or not these are actual subgenres of electro. Electro is considered synonymous to this day with these two genres.
Electro also spawned more valid, genuine subgenres, including electro funk, electro bass, glitch electro, downtempo electro, and minimal electro. Electro also influenced freestyle music and Miami bass (in turn creating techno bass), and had a significant role in the evolution of ghettotech.
When someone refers to electro in modern times it is not likely they are referring to the original hip-hop style. Electro is commonly used as shorthand for describing music from the electropop revival and electro house scene. Electro was extremely influential, inspiring a myriad of subgenres, all of which could be considered electro. This makes it very confusing when trying to ascertain exactly what strand of electro a certain artist's sound fits into.
In general, many people use electro to describe all electronic music meant to be danced to, distinguishing it from electronica (electronic music meant for home listening).
Electro is used most accurately to describe the original strand of funk influenced hip-hop. Electro purists consider this the only valid definition.
Afrika Bambaataa was a pioneer of early electro and electro funk.