In the USA, Second Class Mail isn't used to mail letters; it's a commercial class used mostly to send newspapers and magazines to subscribers.

First class mail would be sealed communications.

Second class mail would be publications.

Third class mail would be advertising and junk mail.

.60 cents for a first class stamp.

.50 cents for a second class stamp.

In the UK the Royal Mail aim to deliver Second Class post within two working days.

Second class post