Future Trunks is the Saiyan and Human hybrid son of Vegeta and Bulma from the future. By the time Present Trunks was born, the timeline had been altered by Future Trunks' and Cell's trips to the past. Therefore, the two Trunks had completely different lives (as opposed to those who lived before the time machines arrived, they lived exactly the same lives as their counterparts until the point at which the two timelines diverge: three years before the androids arrived). "Mirai" is the Japanese kanji name for "future."
This is the first Trunks that is introduced in the Dragon Ball series. Trained by the alternate timeline's Gohan, Future Trunks travels through time to seek help from Goku to prevent the Red Ribbon Androids from ravaging the world, like they did in his dark future. Future Trunks is a loyal and uncompromising warrior, and can be slightly cocky at times. He inherited his mentor's do-or-die attitude, as well as his father's brash confidence (though he displayed less of it at different times; it was most on display in his fight against Frieza). He wields a sword and wears a Capsule Corporation jacket.
Present Trunks as a child during the Majin Buu arc is shown to be quite cocky and stingy, as well as mischievous, most likely due to the fact that this Trunks grew up with his father Vegeta and enjoyed the luxuries of peace, while Future Trunks grew up without Future Vegeta and in an apocalyptic world, causing him to be much more serious and cautious. Trunks tends to usually take control over the younger Goten, stating to him numerous times how he is both older and stronger (during childhood). Trunks at first also has his doubts about Goku, whom he originally believes is not as strong as his father.