I first heard about this show on Wil Wheaton's blog, and shortly thereafter I read an article about it titled something like, 'The Best Show You're Not Watching'. (I would link to it, but I can't find it again now). It is an amazing show, and I agree that not enough people are watching it because I have trouble finding people to talk to about it!
The tag line that comes up with a Google Search for Orphan Black is :
"An orphaned outsider witnesses a woman's suicide and decides to assume her identity."
What lets our orphan, Sarah; assume the woman's identity is their eerie similarity in appearance. Before long Sarah finds out the reason for the similarity is that she is a clone - and there are more out there just like her.
The lead actress, Tatiana Maslany, who plays the clones, just blows me away with her talent. While I'm watching I often forget that these characters are not actually different people, so completely does she give individual personality and mannerisms to each clone. There are many scenes over the course of the series in which one clone steps in to impersonate another, and even then, she doesn't just act like the impersonated clone. She maintains the layers of both, so that it is clear to the viewer that this is not clone B, but actually clone A pretending to be clone B. (I'm trying to avoid getting spoiler-y here. They aren't really called clone A or B, they have real names.)
Maslany isn't the only great talent on the show. The entire supporting cast is fun and diverse and wonderful.
If you are interested in watching, and you should be, this is definitely a show to start at the beginning of season 1. Right now there are 3 completed seasons available, so it shouldn't be too hard to get caught up. I should also probably note that this is not a child-friendly show. Lots of sex and violence.
I confess, I actually wrote this several weeks ago, but was slow in posting it but since I did, now I can add that recently Orphan Black won the Hugo award (a science fiction/fantasy award) for Best Dramatic Presentation Short Form. While I voted for Orphan Black, I didn't expect it to actually win since it was up against episodes of Doctor Who and Game of Thrones which I believed were much more popular. So - Yea!