From the show description :
After witnessing a woman’s suicide, Sarah assumes her identity hoping that cleaning out the dead woman’s bank account will solve all of her problems. Instead, the street smart chameleon is thrust headlong into a kaleidoscopic mystery. She makes the dizzying discovery that she and the dead woman are clones… but are they the only ones? With an assassin hot on her trail, Sarah is racing for answers.
A slightly longer description from another source which sums things up a little more concisely :
Orphan Black follows outsider, orphan and street-wise chameleon Sarah. After witnessing a woman's suicide, Sarah assumes the strangers identity - who happens to look just like her. Expecting to solve all her problems by cleaning out the dead woman's savings, Sarah is instead thrust headlong into a kaleidoscopic mystery as she realizes the dizzying truth - she and the dead woman are clones. As Sarah searches for answers, she discovers the chilling fact that there are more people like her out there - genetically identical individuals who were planted in unsuspecting birth parents and nurtured in completely different circumstances. With no idea who created the clones, she'll need to discover the reason in a hurry as an assassin is killing them one by one.
Tatiana has been on talk shows explaining the cloning aspect of the show so I don't believe it's a spoiler. It's also been given away in some of the previews.
It stars Tatiana Maslany, Jordan Gavaris and Dylan Bruce. Tatiana has been in several Canadian series including "Homeland" and "Being Erica". Don't know much about the other actors.
Apparently it's a BBC America/Space effort and only their second original series after "Copper". Quite liked "Copper", so will be looking forward to seeing how this one fares.