The first adhesive stamps pre-paid by the sender were printed in the UK in May, 1840 - the “Penny Black”. One “penny” stamp was good enough to mail a 1/2 ounce letter anywhere in the UK. 68 million of them were printed in the one year they were used. Around 13 million are believed to still exist - you can buy one for $25 and up.
However - black postmarks didn’t show up well on black stamps, so experiments were made with red ink. Unscrupulous users found out the red postmarks washed off fairly easily, and thus the stamps could be re-used. So the “penny red” was introduced (along with the “twopenny blue”) - both of which were used for the next 35 years.
The original stamps had no perforations, so had to be separated with scissors (or a knife). Perforations were introduced in the 1850s.
The stamps were printed in sheets of 240 - 12 across and 20 down. So one row cost a shilling; one sheet cost a pound. To discourage forgeries - two letters were placed on each stamp - one at the lower left, and one at the lower right. This showed their position on the sheet by row and column - rows were A to T; columns were A to L. A smart postmaster might notice that a series of mail had stamps with the same letters - thus leading to a lazy forger who couldn’t be bothered to use all the letters on his printing plates.
The letter system was discontinued in the late 1800’s (I don’t think any other country used it, except maybe British colonies), but the tradition of the UK being the only country without its name on its stamps continues to this day.