Cecily Cardew is Jack Worthing's ward. She is young, has inherited a fortune from her deceased parents, and she was left to the care of Jack and her governess, Ms. Prism. Although she has money, she is far from "fashionable" since she lives in her country estate rather than in the city,where her ward Jack goes into as Ernest to cause all sorts of havoc. Hence, her character is meant to be a bit provincial, slightly ever more innocent and much simpler in character than Gwendolyn.
Cecily is described by Jack (Ernest) as exceedingly pretty, and with a massive appetite. These very qualities attracted Algernon to her and made her curious to meet her.
Gwendolyn is the daughter of Lord and Lady Bracknell. She lives in London, and her style of dress and language are those usually attached to the upper classes. Gwendolyn is (in the play) slightly older than Cecily, and looks down on her for living in the country.
As different as they are, they also share a lot of similarities:
They are both obsessed about the name Ernest because it is, according to both, the only name a man should have. They also fantasize about love and what relationships should be like. They both write nonsense on their diaries and fantasy stories which they truly believe to be their life stories. They also are scathing and capable of insult as they have a showdown in the garden the moment they meet and accidentally believe that both are engaged to the same man.
Their characters are mirrors of each other, and they represent the views of women on marriage, and how disrupted they are.