Jessica is the outgoing one of the two. She is popular and likes to go out and have a good time. Elizabeth is more relaxed and would rather study or curl up with a good book. Jessica also likes to play the field; whereas Elizabeth is more the monogamous type.
The series chronicles a lot of ups and downs both in the girls' dating worlds and with their parents. Some would probably classify this series as being more young adult, but I do not remember anything being too grown up for us to be reading in upper elementary school. It was just like reading a soap opera, except featuring teenagers.
For the younger generation, another series was created. Sweet Valley Twins featured Jessica and Elizabeth when they were in sixth grade. This features the time in their lives when they first decided they wanted to be different from each other. I didn't like this series as much, because by the time it was written, I was deep into the high school drama.
Apparently other series were created about the girls at a younger age. Sweet Valley Kids was about them in early elementary school. The Unicorn Club featured them and their friends in the 7th grade. Sweet Valley Junior High is about the twins in the 8th grade. Last year, a book called Sweet Valley Confidential came out, that features what supposedly happens to all of the characters as adults. I have never read any of these.
My mother didn't really care for the series and didn't like to buy them for me. I was, however, permitted to check them out of the library and borrowed them from friends.