Elliptic Curve Digital Signature Algorithm or ECDSA is a cryptographic algorithm used by Bitcoin to ensure that funds can only be spent by their rightful owners. It is dependent on the curve order and hash function used. For bitcoin these are Secp256k1 and SHA256(SHA256()) respectively. A public key can be used to determine if a signature is genuine (in other words, produced with the proper key) without requiring the private key to be divulged. In Bitcoin, a private key is a single unsigned 256 bit integer (32 bytes)”]
Source: https://en.bitcoin.it/wiki/Elliptic_Curve_Digital_Signature_Algorithm