There are a lot of different Mills and Boon genres. You can have your contemporary modern books which usually feature tall, dark and handsome and extremely rich heroes who are paired with a variety of different types of heroines from the innocent pure stay at home types, to modern independently wealthy women. These are the books that are commonly viewed and disparaged as mills and boons.
You also have historical romance, paranormal romance, tender romance, clean christian romance, medical romance.
www.millsandboon.co.uk is the best place to check out the different books.
www.eharlequin.com is their parent company which has a larger variety