A cryptanalyst can choose arbitrary plaintext to be encrypted and get the corresponding ciphertext. He tries to acquire a secret ciphering key or to create algorithm for decrypting all ciphertexts encrypted using the same key.
It is a comfortable situation for the attacker. He can obtain more information about the secret key and about the whole attacked system, by choosing for encrypting texts which were created and selected by him.
During breaking deterministic ciphers with public key, the intruder can easily create a database with popular ciphertexts, for example with popular queries to the server. After that, to find a meaning of the given ciphertext, he can simply check the database.
Chosen-plaintext attacks were used by Allied cryptanalysts in World War II for breaking of Enigma ciphers.
In this kind of chosen-plaintext attack, the intruder has the capability to choose plaintext for encryption many times. Instead of using one big block of text, it can choose the smaller one, receive its encrypted ciphertext and then based on the answer, choose another one, and so on.