He is the Servant of one of the series' main heroines, Rin Tōsaka. Due to an incomplete summon, he apparently has no memory of his previous life or identity. Archer is sarcastic and cynical - but under his hardened exterior lies a complex and dark personality that unfolds throughout the story. Though skilled with ranged weapons, he can hold his own in melee combat. Archer considers Shirō Emiya's idealism to be weak and delusional, recognizing Shirō's mentality of "wanting to save everyone" as naive and impossible.
(Source: Wikipedia)
Archer is actually the future version of Emiya Shirou, who became a superhero through a wish to the world. Archer is skilled in archery through Shirou's own talent in the field, swordsmanship through training his body for his optimal fighting style, as well as some skill in magic through Shirou's training. Archer became a Counter Guardian when he prayed to the world for a miracle saving about 100 people. He was then scapegoated by one of the people he saved, and executed. Initially fighting for his ideals, Archer is eventually forced to realise the truth that everyone cannot be saved, and proceeds to make the minimal sacrifices for the greatest amount of people saved. Feeling betrayed by the world, Archer prays that he is summoned in the Holy Grail War so that he can correct his mistake through a paradox and kill Shirou to stop him from becoming a Counter Guardian.
(Source: Type-Moon wiki)
