Josei manga is shoujo manga's big sister: while shoujo means girl, josei means woman and josei manga are published in magazines marketed to women aged 18-30. Of course many of these series are of interest to older teens, and many shoujo manga are geared to young women: there is a great deal of overlap.
Josei manga has a different visual and storytelling style from shoujo manga. You may be surprised at the simple, stripped down art and closer resemblance to Western panel layouts. AnimeFringe describes josei manga as a "genre of manga [that] tends to deal with much more adult aspects of being a woman in contemporary Japan, such as frank depictions and discussions about sex, dating in today's world, and being in the workforce."
OT = Older Teens. M = 18+