A sapient species, from Earth.
Humans independently discovered a Prothean data cache on Mars in 2148, and became aware of the mass relay networks shortly thereafter. They have expanded rapidly, both in terms of territory and influence -- some would say too rapidly, as many of their problems both practical and political stem from this rapid advancement.
Humans are roughly on par with turians physically (as a human in or around peak physical condition can overwhelm a similarly fit turian, making them near equals in a hand-to-hand combat situation) -- though lacking a metallic carapace might hinder them a bit. They're less agile than asari, though a human male is probably on average physically more powerful (on the other hand, Biotics are the norm in asari and rare in humans). Humans would appear on average to be stronger than salarians but not as fast, due to the extremely elevated metabolisms of the latter.
Alpha Centauri (formally "lost" colony, shared with a primitive native race)
Anhur (shared with batarians)
Zast (technically an illegal colony)