I used to have the biggest collection of Barbie dolls when I was a wee lad.
what type of doll is Barbie dolls?
Barbies are a fashion doll created by Mattel.
what are they used  for?
They are toys or collectibles. They've been a critical part of the toy market for over 50 years.
where can i buy them ,i think my kids would love it?
You can buy them in any toy store. I would buy them soon, since Barbie sales have been declining since 2014.
do they come in different types and colors?
Barbie
Barbie is a fashion doll manufactured by the American toy company Mattel, Inc. and launched in March 1959. American businesswoman Ruth Handler is credited with the creation of the doll using a German doll called Bild Lilli as her inspiration.
Barbie is the figurehead of a brand of Mattel dolls and accessories, including other family members and collectible dolls. Barbie has been an important part of the toy fashion doll market for over fifty years, and has been the subject of numerous controversies and lawsuits, often involving parodies of the
Fashion doll
Fashion dolls are dolls primarily designed to be dressed to reflect fashion trends. They are manufactured both as toys for children to play with and as collectibles for adult collectors. The dolls are usually modeled after teen girls or adult women, though child, male, and even some non-human variants exist. Contemporary fashion dolls are typically made of vinyl or another plastic. Recently, 3D software versions have appeared. 
Barbie was released by the American toy-company Mattel in 1959, and was followed by many