Size and Weight:
First things first: although Smilodon is referred to by most people as the saber-toothed tiger, this prehistoric mammal wasn't a true tiger at all, belonging instead to an ancient, long-extinct line of cat-like creatures known as "machairodonts" (a related genus, unsurprisingly, was Machairodus). Other than that, though, what you've heard is mostly true: Smilodon was a large, muscular predator that may well have snacked on early humans as well as the Woolly Mammoths and Giant Sloths of the Pliocene and Pleistocene epochs. (See 10 Facts About the Saber-Tooth Tiger.)
As formidable as it was--and despite what you've seen in the popular media--Smilodon didn't hunt like a modern jaguar or puma. Like other saber-toothed cats (and "dirk-toothed" and "scimitar-toothed" cats as well), Smilodon would leap on its prey suddenly from the high branches of trees, digging its huge incisors into the unfortunate animal's neck and then withdrawing to a safe distance while its dinner bled to death. It's unknown whether Smilodon hunted in packs, though that would certainly have helped it to take down massive herbivores like the giant elk Megaloceros or the giant, prehistoric cow known as the Auroch. Smilodon may even have scuffled with the Dire Wolf, Canis dirus; see The Dire Wolf vs. the Saber-Tooth Tiger - Who Wins? for an analysis of this epic battle.
In case you're wondering why Smilodon has appeared in so many movies, that may be because thousands of intact Smilodon skeletons have been extracted from the La Brea Tar Pits in Los Angeles, a stone's throw from Hollywood (the California variant of this genus, Smilodon californicus, is the official state fossil). By the way, the last specimen of Smilodon went extinct only 10,000 years ago; by then, primitive humans had figured out how to hunt cooperatively and killed off this dangerous menace once and for all (it also didn't hurt that these same humans also hunted to extinction the giant, herbivorous megafauna that Smilodon snacked on).