One of the largest land animals ever to have existed, an adult brachiosaur could weigh roughly 80 tonnes (for those wondering how to make sense of this, it's about as heavy as 20 elephants). Its name means "Arm lizard", because of its unusually long and heavy front limbs, much bigger than its hind limbs, which meant that it had a strangely sloped back while standing up.
Until relatively recently, common wisdom held that Brachiosaurus spent most of its time in deep marshes, grazing on floating weeds, using the water to help support its immense weight. However, this was based on the theory that dinosaurs were cold-blooded like other reptiles, an idea which has been losing ground recently as evidence has emerged that they may have been warm-blooded and therefore far more energetic that was previously thought. Current thinking is that Brachiosaurus may have had a lifestyle similar to that of a modern-day giraffe, grazing from high trees and roaming in herds.
Another unusual feature of this dinosaur, which contributed to the belief that it was a water-dweller, is the placement of its nostrils on a bony crest above its head (which would allow it to remain almost completely immersed for long periods). It is now thought that this simply helped to give it a powerful sense of smell.
Brachiosaurs were members of the Sauropod family, and lived in the Jurassic period, roughly 156-145 million years ago.
Some information from: http://www.bbc.co.uk/dinosaurs/fact_files/scrub/brachiosaurus.shtml
For all the Brachiosaurus links you'll ever need: http://brachiosaurus.allbio.org/