Mail is part of the postal system. The postal system is a system where written documents and small to medium packages are sent (delivered) to places around the world. Anything sent through the postal system is called mail or post.
A postal service can be private or public. Governments often make rules about private postal delivery systems. Since the early 19th century, national postal systems have mostly been controlled entirely by governments. The governments make people pay a fee to send something. This fee is usually in the form of stamps. Some governments only let private postal services deliver parcel (non-mail) packages.
Postal systems often do things other than sending letters. In some countries, the postal system is also in charge of telephone and telegraph systems. The postal systems in other countries can help people apply for government papers such as passports and driver's licenses. Many places the post offices also work like banks. This is so that people can take out money or pay bills at a nearby post office, instead of travelling to a big bank in the nearest town.
People drop letters in a post box to be collected and delivered by the post office.