P1. Green Screen

What a green screen does:
A film technique / special effect which involves actors or objects in front of a block green background. This then gets edited on a software such as Adobe After Effects, where computerized graphics can be added,  such as backgrounds which aren't actually there.  This is used to create artificial backgrounds so they can film in the studio, which is more efficient as they don't have to travel saving time and money. It can also be know as a way to combine different shots and elements into one image.

The History
The first film to ever use green screen/chroma key technology was 'Four Heads are better than one' in 1898, directed by Georges Méliès. The most common colour used in chroma key technology was previously blue, but as the years went on, they realized the colour green was a better. Some of the reasons for the colour change was because digital cameras came out and where more commonly used and the green channel is the cleanest in most digital cameras today and has the most luminance, resulting in the least noise. Another reason was because blue is more commonly worn as costume, and green screens permit much less light.. In 1970 they finally decided to change the screens from blue to green, which then became widely used and famous for video editing as well as photography. 

Green Screen used in Avengers

In this scene, they green screened the background in, which worked  well and is extremely convincing as the separate footage was carefully matched.
The reason they did it was because in the scene there had to be broken skyscrapers, cars, buildings, fire and smoke, which would have cost a lot of money to build on set and wouldn't of been practical.  Also, because it was located in a city, they couldn't film on location, therefore green screen was the only option.
They did this by using a green screen in the background and later on adding in an image/painting where the screen was. This also allowed them to add smoke and fire over the top.
The reason I find the green screening effective is because the character actually looks as if he's there, due to the lighting match, colour match and realism of the painting/photo.