The system of international maritime signal flags is a way of representing individual letters of the alphabet on ships or in nautical situations.
The flags can also be used to signal other things than alphabetical messages. For example, divers will often raise the flag representing the letter A as a marker to other sea traffic in the area.
The flags are also used, with a different set of special meanings, in yacht racing and dinghy racing, where (for example) the P flag is used as the "preparatory" flag to indicate an imminent start, and the S flag means "shortened course".
External links
The flags and their meanings (http://www.anbg.gov.au/flags/signal-meaning.html)
Flags Fantastic - Nautical flags (A - Z & Pennant) (http://www.flagsfantastic.com.au/html/nautical.html)
La flag-alfabeto (http://lingvo.org/flagoj/) - signal flags used for the Esperanto language.
The flag of Nepal is the only national flag which is not rectangular, being based upon two separate pennants which belonged to rival branches of the Rana dynasty, which formerly ruled the country.
The pretty uniform shape of the national flag can be probably explained by the fact that the national flag has its origin in a limited area (Europe and Mediterranean), as shipflags.
National flags of countries outside Europe only developed after European national flags had standardized their shape, so they were imitating the rectangular shape as well as some of the symbolic elements (vertical/horizontal stripes, for instance).