Can someone tell me how cell phone plans work in South Korea?

In the United States, one buys a cell phone, and then signs a contract (usually for a year) with a provider for the service. He pays a per-month fee to keep the service going. The number is fixed, as is the commitment to pay the per-month fee. In other words, he is stuck with his number and the phone for the contract period.

In Thailand, one buys a cell phone, and then he buys a special chip that has a phone number encoded in it that allows him to link with the service provider. The chip is installed in the phone, and the owner buys debit-type phone cards to continue service. No contracts are signed, and one can change his number anytime he wishes by simply buying a new chip.
He can also opt out of service by simply not recharging his account when he runs out of time on his account.

Can someone tell me how cell service works in South Korea?

Thanks a bunch,

Mangy Dog