The Basics
Each account has one of the two pairs of private and public cryptographic keys, with the private spend key used to spend any funds in the account.
In-depth Information
The private spend key is a 256-bit integer used to sign Monero transactions. With the official wallet’s current deterministic key derivation method, the private spend key is also an alternate representation of the mnemonic seed. It can be used to derive all other account keys.