Seal Certificate(인감 증명서)
You can register your personal seal at the Dong-Office(동사무소, 주민센터) with your ID, ARC and Passport.
This seal certificate is a kind of Notarized Signature Certificate, but you can get this certificate only by 600won(50U$cents) at any dong-office once you registered.
We used to attach this certificate while putting my seal on an legal document especially a contract for dealing house, car, loan agreement or endorsement for guarantee as well as on the legal liability documents at the tax office, court, bank or government application.
You can register this personal certificate at the Gu-Office(구청) only, but from this April, you need to register at the Dong-office where you're registered as your residency.
This certificate is used to be expired in 3 months but it depends on situation and events you need this certificate.
Like I said, you can also ask to issue the "Signature certificate(본인서명사실확인서)" but it's not usual yet.
For the "Seal(도장)", you can engrave Alphabet, or Korean(한글), and I always recommend to engrave your full name, for example, Ronald Reagan, saying in Korean, 로널드레이건 to easily recognized it by a Korean. You can also use this seal for the bank.
I used to recommend you guys to register this personal seal certificate, since I can use it as your immediate PoA(Power of Attorney) with a reasonable cost for a various red-tape-job in my country.