COS is short for "Costume" or "costume play." Think the kind of dress-up that you'd find at a Renaissance Fair or Science Fiction/Fantasy convention. You will find the a bunch of girls dressed up like Princess Leia in Star Wars (especially the outfit she wore when she was being held as a sex slave to Jabba the Hut) but, more commonly, you'll find people dressing up as their favorite character from some Japanese comic book or cartoon.
While there are lots of young men dressing up, most of the pictures you'll find will tend to be of the pretty girls. Some look young - very young - especially with the little girl outfits they wear. While I am sure that there are a goodly number of adults dressing up in costume, it sometimes can be hard to tell ages. Many of the under-aged ones seem to stress their sexuality, making them appear older than they really are. On the other hand, there are a lot of adult women dressing like under-aged girls. While I would never endorse pedophilia, many of these girls are nothing more than cock teases having fun. I wish that was one of them!
From the sheer number of Asian girls in these images, it is easy to image that COSPlay is much more commonplace in the far east. In the Americas, you'll find the most COSPlay at comic book and sci-fi conventions such as ComicCon. At these American events, these are more costumes from science fiction movies and fantasy games than comic books, but the gals still seem to be having loads of fun.
Strictly speaking, COSPlay can be found all over. Take a look at any high fashion or even fetish pictures and you'll find lots of costumes.
When I was little, I always wanted to dress up like Yvonne Craig as Batgirl or Diana Riggs from The Avengers - I guess, even then I had a thing for skin-tight leather cat suits.