Each set of private keys is capable of generating billions of public keys. These public keys are then transformed (through a mathematical process called hashing) to produce public addresses. Every single one of those addresses can receive Bitcoin. So each set of private keys you own is capable of producing its own unique, massive set of public addresses that you, and you alone, own. Anyone can send Bitcoin to those public addresses, but only the holder of the private keys can spend Bitcoin from those addresses.
Many people like to analogize private keys, public addresses, and Bitcoin wallets to email addresses. The private keys are your password, the public address is your email address, and the wallet is the email client you use (Gmail, AOL, yahoo etc…). Although useful, this analogy is slightly misleading because with Bitcoin each password (private key) that you own gives you access to billions of potential email addresses (public addresses) to send Bitcoin from and receive Bitcoin to.
So don’t be concerned if your wallet consistently generates new Bitcoin addresses. That’s actually one of its features! Wallets generating and using new public addresses helps protect your privacy from people snooping on the public Bitcoin blockchain. Just remember that so long as you still hold the private keys to your wallet, you alone still own the present and future Bitcoin sent to any public address that your wallet generated. Neither wallet addresses nor balances can expire. Just make sure to keep your Bitcoin private keys safe, secure, and private.
For more information about Bitcoin wallets read here