Lady Catherine de Bourgh is the aunt of Mr. Fitzwilliam Darcy. She is the owner of Rosings Park, and is the arch-nemesis of Eliza Bennet.
We first meet her when Elizabeth comes to visit Mr. and Mrs. Collins, neighbors to Lady Catherine and her daughter. During the visit, she invites Lizzie and family to dine often, but makes it unpleasant, because of her condescending attitude towards everything and everyone in life. For example, she frequently gives the worst food possible to the poor, and keeps the richest meat for herself.
Also, she has a strict sense of where everyone belongs in society, and is shocked when someone disagrees. For example, when Eliza states that her sisters are all "out", Lady Catherine is surprised, and replies that it is not proper for all sisters to be out at once.
We don't see her very often after Elizabeth leaves Hunsford, although we hear from her often. The next time we DO see her, is when she comes to visit Eliza in her home, in order to confirm a scandalous rumor about Mr. Darcy and Elizabeth Bennet. This sparks a fight when Elizabeth refuses to be domineered by Lady Catherine, and Lady Catherine is not used to having people talk back to her, especially 18-year-olds. She leaves, and is so upset with Mr. Darcy's and Elizabeth's decision that she refused to talk to them for months, and only concedes when her nephew contacts her. After she leaves Longbourne, we never see her again.