Impressionism history defines French impressionism as follows:
In impressionistic paintings visual reality loses it's corporality and thus becomes only an appearance, an impression.
Impressionistic painters don't show their motive as it is, but how they see it. The process of perception
becomes the image and is no longer what is observed. The medium with which this process is made visible is light. The impressionists wanted to render the effect of light by using pure colors.1
Does this tell you anything? I don't blame you if it doesn't, because you interpret the above statement against your contemporary background. You compare impressionistic paintings with our current perception of how paintings are made, in which the above criteria have become a matter of course, as will become clear.
The above definition of impressionistic paintings makes much better sense if you put yourself in the shoes of a young 19th painter who is looking for ways to free his paintings from the strict classical figurative rules. Then it's easy so see what the above says: To copy the observed literally is no longer necessary, the artist may interpret and represent his subject as he sees fit.....within the limits that the impressionists respected.
From our perspective impressionism was "just" one of the first attempts the break with figurative rigor and to us the above-mentioned limits (within which impressionists sought freedom at the expense of figurative accuracy) are more meaningful and descriptive of impressionistic paintings. But art historians today define impressionism in a 19th century context and expect the layman to be aware of this. This is one way of understand impressionism.
Another approach is to define an impressionist as an artist that only paints what the eye sees and not what the artist knows. In that sense it can be useful to compare impressionism with cubism in which "the essence of things is more important than their appearance".
Cubists like Picasso and Braque would paint their subject as if it were seen from different viewpoints simultaneously and thus show more (in a sense) of the subject than could be seen at one moment. Then what the artist knows about his subject takes precedence over what he sees, the reverse of impressionism.
The term impressionism was invented by art critic Louis Leroy at an 1874 exhibition when Monet was asked about his painting "Impression, sun rise" (1872).
For the catalogue they wanted to know what the title of the painting was, because it couldn't really pass for a view on Le Havre. I answered: "Just call it impression."
On the technical side the best-known technique innovation is perhaps pointillism in which paintings consist of a multitude of dots in pure color. As in impressionism it's the impression that counts, the impressionists often didn't bother to carefully finish their works as this would interfere with a painting's spontaneity, diminishing the truthfulness of the impression. They tried to incorporate photographic effects such as the blur and different contrasts that photos can have. As opposed to the academic rules of perspective, some impressionists (like Manet) favored a "natural" perspective.
1 . After "Timelines - Die Kunst der Moderne", Sandro Bocola - Benedikt Taschen Verlag GmbH