The Metroid was created long ago by an ancient tribe known as 'The Chozo', in order to fend off a threat known as the X Parasite. The Metroids were discovered on Planet SR388 years after their creation. After the Space Pirates discovered the Metroids, they were promptly used as bioweapons, due to the Energy Leeching abilities the creatures possess.
In the original Metroid, Samus was sent to Planet Zebes to destroy the Space Pirates, as well as dispose the stolen Metroids. In Metroid II: Return of Samus, the Galactic Federation noted the danger the Metroid posed, and called upon Samus to go to SR388 to eradicate the Metroids once and for all. During the last battle with the Queen Metroid, and infant Metroid hatched and imprinted Samus as her mother. This last Metroid because known as 'The Hatchling' and would become one of the most important Metroid in the series.
In Super Smash Flash 2
A Metroid is a life form that feeds on the Life Energy of their prey. It returns as an Assist Trophy from Super Smash Bros. Brawl and as such, anyone who played that game will be familiar with the effect of the Metroids. After being released, it will fly around, and eventually latch onto the head of one of the opponents. Once latched, the Metroid will start draining Life Energy, racking up the damage meter of the opponent. Movement is also hampered when a Metroid latches onto a character. The Metroid appears and acts exactly like it's appearance as an Assist Trophy in Super Smash Bros. Brawl.
Note, that Metroid can be attacked in Super Smash Flash 2.