What is the meaning of "probabilistic encryption algorithm"? I am learning cryptography by myself, and I am not able to understand the meaning of "probabilistic encryption algorithm".   

How is it different from "deterministic encryption algorithm"? How can the output of a probabilistic algorithm be different for the same pair of plain text and key when used two different times? Doesn't the cipher text depend only on key and plain text?
Informally, in probabilistic encryption random values are used to encrypt a message. Thus, each time we encrypt a message we pick a fresh random value; as a result if we encrypt the same message twice we would get different encrypted value (or ciphertext).This means that the ciphertext does not depend only on key and plaintext.