Borderlands didn't trot out a reboot or a prequel for its latest edition; Borderlands: The Pre-Sequel is something in between, and if you're wondering how that fits into the continuity — and can tolerate a lot of spoilers — here's a visual timeline.
I suppose it's not a spoiler that Claptrap "spends most of [Borderlands] standing next to a frickin' bounty board." His brethren also spend much of that time missing some part deep in a cave populated by crabworms and bandits. This timeline also does not discuss the relationship of Lucky Zaford to Scooter's Mama in the original game, either.
We weren't as sweet on the latest Borderlands as we were on its, er, younger? sibling Borderlands 2, scoring the Pre-Sequel a 7.0, calling it "a story filling in gaps," but adding that the low-gravity influence of the moon environment makes it worth playing.