What are those flying in our sky?

First of All Know these Terms:
Civil aviation: It is one of two major categories of flying, representing all non-military aviation, both private and commercial. Most of the countries in the world are members of the International Civil Aviation Organization (ICAO) and work together to establish common standards and recommended practices for civil aviation through that agency.
Commercial aviation: It is the part of civil aviation (both general aviation and scheduled airline services) that involves operating aircraft for hire to transport passengers or cargo.

Route map of the world’s scheduled commercial airline traffic, 2009

The freedoms of the air are a set of commercial aviation rights granting a country’s airlines the privilege to enter and land in another country’s airspace, formulated as a result of disagreements over the extent of aviation liberalisation in the Convention on International Civil Aviation of 1944, known as the Chicago Convention.

Freedom Of the Airs:

Freedom Description
1st the right to fly over a foreign country without landing
2nd the right to refuel or carry out maintenance in a foreign country without embarking or disembarking passengers or cargo
3rd the right to fly from one’s own country to another
4th the right to fly from another country to one’s own
5th the right to fly between two foreign countries on a flight originating or ending in one’s own country
6th the right to fly from a foreign country to another while stopping in one’s own country for non-technical reasons
7th the right to fly between two foreign countries while not offering flights to one’s own country
8th the right to fly inside a foreign country, continuing to one’s own country
9th the right to fly inside a foreign country without continuing to one’s own country

What are those planes leave a trail behind them?

Jet planes fly at very high altitudes, often at 26,000 feet above sea level or more. At these heights the air is very cold, -40oC or less. As a jet plane flies at this height, it releases hot water vapour and carbon dioxide from its engines. This causes water vapour to condense immediately into water. These trails are caused by condensing water vapour.

What are Jet Plane For?

As in fighter aircraft. They are used for training for a world war and airshows.

Freedom of air are for all non-military aviation. Nepal is also a member of International Civil Aviation Organization (ICAO)  Now ask Yourself:
What are those flying in our sky?