A postal code is a string of letters and/or digits appended to a postal address for the purpose of sorting and routing snail mail.
In various countried it is know as a post code, postcode, or ZIP code. Currently more than 120 countries have own postal code systems.
Surprisingly, some pretty big countries like Panama or Ireland appear not to have national postal code systems though, what makes it a tough challenge to provide a postal code finder for such countries.

Usually postal codes are assigned to geographical entities, such as cities, counties, regions.
Meanwhile some countries, for example France, may have special postal code range for institutions or even standalone addresses, usually for those who have large volumes of mail, like large international companies or goverment authorities.
This type of codes structure may ease the use of postal code search in various applications.

I bet you will be surprised to know that first time in the world a sort of postal code system appeared in Soviet Russia in 1932, then in more than 10 years Germany followed suit, and in 1958 Argentina joined the club. Western countries, such as United Kingdom and United States first introduced own zip code system in 1960s.

Usually a postal code is comrised of arabic numbers from 0 to 9, basic Latin alphabet letters, possibly a space and a hyphen. In Ukraine they may also use cyrillic letters. A space and a hyphen are for formatting purposes for human eyes, modern computirized postal code finder systems are ignoring these.
Also, some countries like Canada and the Netherlands do have some reserved characters that are not used in codes, and a postal code search interface is not accepting such letters.
The majority of countries including United States, Russia and others use numeric codes, i.e. postal codes are comprised of only digits, while United Kingdom, Argentina, Jamaica, Netherlands and others, use alphanumeric rules, i.e. digits can be mixed with letters.
Also some countries with advanced hitech postal systems prefix their national postal codes with a country ISO prefix, for example LV for Latvia or EC for Ecuador. This is a first step to make global postal code search engine possible.

Regarding the placement of the code, it considerably varies from country to country.
In English-speaking countries, the postal or zip code makes the last item of the address, just after the town or city name.
In continental Europe, though, the postal code tends to precede the name of the town or city. In Asia, like China, Japan, or Russia, the postal code is the first thing to start the address.
Taking such varietes in mind, one can imagine the difficulties to create a general postal code finder computer software application.

Precision of postal code system also varies from country to country. For example, in the Netherlands, a postal code contains information up to the house number – imagine that street names are redundant in this situation. United States also may have house number in the zip code, while United Kingdom offers linking to a road. In other countries, such as Russia, it is less that specific, and a postal code is assigned to a town district or a rural area district.