RC4 and Stream Ciphers

WEP protects the confidentiality of its data using a stream cipher called RC4.  This is a well-known cipher, and the designers were wise to choose it instead of trying to make their own.
With a stream cipher, you generate a keystream K and XOR that keystream with the plaintext to get the ciphertext
The receiver generates the same keystream, and XORs the ciphertext with K to get back the plaintext

This works because XORs cancel:
P' = C XOR K = P XOR K XOR K = P
So the message that was received is the same one that was sent