On Solana, programs are stateless. If your program needs to remember something per user, per game, or per configuration, it needs a deterministic address it can find again later without storing it anywhere. PDAs are that address.
What a PDA actually is
In Web2, a database primary key is computed from a row's logical identity — user_id, order_id, something the system already knows. You never look up a row by a random UUID you can't derive. PDAs work the same way, except the database is the entire Solana account model, the key derivation is a hash, and the program ID is baked in so only your program can sign for it.
Unlike a primary key, a PDA is not stored in a table. It's derived on demand and may or may not have an account at that address yet. The program doesn't know if the account exists until it tries to read it. That's why init and init_if_needed exist.
Anatomy of a derivation
