Definition of Latitude and Longitude

The coordinate system that we use to locate places on Earth is the terrestrial system. The coordinates in the terrestrial system are Latitude and Longitude.

 

Longitude

Longitude, denoted symbolically by the Greek letter Lambda, is divided in meridians (not parallel to each other, they converge at the poles), which are measured in degrees East or West of the Prime Meridian, also known as Greenwich Meridian (they are the same thing).
The Prime Meridian serves as a starting point for the measurement of degrees in either East or West directions. It marks longitude 0°.
Another meridian of great importance is the Dateline Merdian, which marks longitude 180° (either E or W). This meridian is the exact oposite of the Prime Meridian on the globe. 

In case you are wondering...

 Why do we use Greenwich to set where zero longitude is located?

At the time when European sailors began to travel around the world in the 16th century, England was very active in colonization. As a result, it had many ships sailing the seas and was very susceptible to lose ships due to poor navigation (orientation). Longitude could be measured accurately on land by (a) use of a very accurate pendulum clock and (b) by observation of rare astronomical events to set the clock to match one at home. However, a pendulum clock does not work well on a ship.

The Royal Greenwich Observatory (RGO, now a museum, near London) was established with the purpose of finding a way to solve the longitude problem for ships. The RGO established its "meridian" at the place where the main instrument for timing stars was built . The Paris Observatory had its own meridian and French sailors preferred to reckon longitude from there. Yet, the English managed in the end to solve the longitude problem and gained the Prime Meridian.
What is the Dateline, and what is its use? If you travel from one side of the Dateline meridian to the other... It is a different day! It simply makes things work better.
- This means that when you fly from the right side (Western Hemisphere) to the left, you lose one day: Began travelling from Canada on Monday, arrived in South Korea on Tuesday, despite this being a trip that lasted only about half a day.
- This also means that if you fly from the left side (Eastern Hemisphere) to the right, you gain one day:
Began travelling from China on Friday, arrived in Mexico on Thursday, despite this being a trip that lasted about half a day.