Slight correction of the correction.  ECDSA is not an encryption algorithm.   It is a digital signature algorithm.  It is not possible to perform encryption or decryption with ECDSA only digitally singing and verifying signatures.

Private keys are used to sign Bitcoin txs to "prove" the tx was created by someone with access to the private key.
Public keys are used to verify Bitcoin txs are properly signed by the correct private key.

There is NO encryption in the Bitcoin protocol although some clients do employ encryption client side to protect wallet files from unauthorized access.

Cryptography
 - Hashing Functions
 - Digital Signatures
 - Encryption

All encryption involves cryptography, not all cryptography involves encryption.