The concept behind Roswell is this (attempting to avoid spoilers):
Aliens crashed years ago in Roswell. But a trio of human-looking alien hybrids were kept in stasis.
These children broke out of stasis and have been adopted by human parents.
Now, dealing with teen angst, wondering where their true home is, and discovering unusual powers and abilities, they try to fit in to life in High School while keeping their identities hidden from government agencies and alien hunters...
Lots of similarities to Smallville (although not quite as good, but still very enjoyable). For the price, it's definitely worth it. I found it intriguing from episode 1, and it really picks up the further you get into it.