The Protheans are an extinct race in Mass Effect that existed 50,000 years prior to the events of the series.
Protheans are humanoids of unclear ancestry. Their scaled heads are flat and tear-drop shaped, with the point at the back. They have four eyes with yellow irises and goat-like pupils. Protheans have three fingers (including an opposable thumb) on each hand and two toes on each foot.
Protheans are sensitive to the biochemistry used by humanoids to store memories, allowing them to share memories with other sapient beings through physical contact. They can even pick up some information from residue left behind by other beings on an object.
The Prothean civilization was a Roman-style empire, with the Protheans ruling an unspecified number of subject races. The center of Prothean government was the Citadel space station. Subject races would eventually identify themselves as "Prothean", regardless of their original self-identification.
At one time the Protheans dominated the galaxy, only to be eradicated by the Reapers upon their return through the Citadel. Although warned by their own precursors, the Prothean strategy for defeating the Reapers depended on all races in the Empire following a single doctrine. When the Reapers identified weaknesses in this strategy, the various races were unable to adapt.
A small Prothean research team survived the genocide, and -- once the Reapers left -- they returned to the Citadel, sabotaging it to make it more difficult to bring back the Reaper fleet. This gave the next generation of spacefaring races the chance to delay the Reaper invasion rather than being taken completely by surprise.
The Reapers hid all traces of their involvement in the Prothean downfall, leading the later spacefaring races to assume that it was the Protheans who built the Citadel and the mass relay system. In reality the Reapers built them and the Protheans simply found them.
The Reapers used the Protheans to create the Collectors.
The Protheans had more advanced technology than the current Council races in multiple fields, and they controlled almost the whole galaxy. Little else is known about their capabilities.