Mr. Darcy and Elizabeth??

We have recieced both a letter from Mr. Collins and a visit from Lady Catherine. Both of these events were concerning the relationship between Darcy and Elizabeth. Lady Catherine came to tell Elizabeth that her daughter Miss de Bourgh is to marry Darcy and Elizabeth is not. Lady Catherine also tried to make Elizabeth promise to not marrying Darcy and she said no!! What is going on? The letter from Mr. Collins said that Lady Catherine is mad because Elizabeth is going to be married to Darcy. This is absurd.