Wickham is charming in public. He makes an effort to be agreeable. He cares greatly about what others think of him. He puts on pretenses to please whoever he is around. He is also, underneath that socially amiable veneer, a scoundrel. He uses people for their money, wastes his own money, and preys on vulnerable women to satisfy his own desire for money, or other pleasures. He is shallow, vain, and selfish.
Darcy, on the other hand, appears to be aloof and snobbish in public. He doesn't go out of his way to appear pleasant, or to make connections. He is awkward and shy, and uncomfortable around those that he does not know well, or when he is not in an environment where he is comfortable. However, once you get past that exterior, he is a very loyal, moral, upright man who does what is right. He is full of integrity, discretion, and selflessness. He protects and loves his sister, helps Elizabeth in a time of need without bragging about it, and is kind and attentive to her aunt and uncle. He has a sense of duty and propriety, is respectful, and a good friend and landlord.
Elizabeth's judgment of both of these characters is both accurate and false. She is pleased by Wickham's charming and amiable nature, and indeed he does have one, so she is accurate. She is put off by Darcy's social aloofness, and concludes that he is a snob. While he isn't a snob, he is awkward socially, so she picked up on that a little bit. However, in terms of their true characters, Lizzy's judgment of them is flip-flopped; she deems Darcy the immoral scoundrel and Wickham the knight in shining armor. We find out later that it is actually the opposite. And, her misconceptions there are the basis for the entire conflict in the novel. What would we do if Elizabeth hadn't misjudged? :) It would've made for a boring novel, indeed.