Impressionism is the direct result of Modernism; one cannot claim to understand the former without the latter. The late Victorian ideas of "the painter of Modern life," such as were described by Charles Baudelaire and Édouard Manet, found their manifest culmination in this new style of art which ultimately grew to become viewed as the chief hinging point in the progression toward Modern Art (people today largely regard it in this way). The movement took inspiration from Manet's paintings, but since Manet himself never fully committed to a single art style, his work remains detached from the label of Impressionism. Although he directly influenced this new period of art, Impressionism, strictly defined, begins with Claude Monet.
Monet was another French artist who had been classically trained according to the traditional style of realist art, but his training included a technique called "plein air" painting, which is painting done outdoors. (His subjects were mostly, then, landscapes). In this technique, the artist brings his canvas, easel, and palette to the actual spot and paints from the immediate perspective of visual reading. However, Monet discovered something. Nothing brings one closer to nature than plein air painting, since it offers the artist the experience of physical presence within its environment, but with the added proximity comes problems. For starters, the sunlight changes too rapidly to paint a landscape in the careful, detailed manner in which artists were trained to do in the 1800s. The weather also changes; wind can distort the clarity of tree branches, wispy clouds, and other natural elements; and there are active participants in the scene who frequently interrupt the serenity of the landscape, such as animals or people. One does not so much paint nature as experience it when painting en plein air. Therefore, Monet's painting style needed to change; it needed to be a technique which could keep up with nature's active movement and changeability. And the new method was: paint faster!
Haha, not exactly; but kind of. Monet used quick, short brushstrokes and small dabs or spots of color, blended together, to try to capture the immediate image of a natural landscape before it changed. Specifically to capture the visual picture of what the eye sees at a given moment out in nature, he shifted his focus more to atmosphere and less on detail. In that way his paintings became mostly about capturing the effects of light and shadow on objects, since these were the most rapidly transitory. Bad weather could be avoided, and birds and other creatures were not too much a nuisance, but the sun always seemed to set too fast. If he began a painting in daylight and remained at the scene until nightfall, he would have to start an entirely new painting, since the scene before him had totally changed. His quick paintings, then, were an effort to capture an impression of the brief, passing moment of what the eye sees in nature, like a snapshot.
Thus in 1873 Monet produced this work, which he titled Impression: Sunrise. Upon the artist's submitting this work at the Paris exhibition, critics saw it and were outraged. They complained it was rough and unfinished, and they mocked the title by calling it only an "impression" of art, not to be considered real art. Instead of feeling inferior at this insult, the artist adopted this negative label as a badge of honor and called himself thereafter an Impressionist. Claude Monet is famously reported to have said, in response to the news that his artwork was being scorned and mocked by the salon art critics, "What do the critics know?"
You can see that in this painting realistic detail plays almost no role at all in presenting subject matter. The landscape is shown through the visual effects of elements within the natural environment. The rising sun appears red through the smog of a harbor and causes the entire sky in the scene to appear fiery and dirty. We cannot even see the harbor, it is so covered by smog in the distance. There is no horizon line, and the reflection on the water's surface causes the distinction between land and sea to blur and almost disappear. Since the sun is brightest, it and its watery reflection are given the clearest brushstrokes. The boats in the foreground also appear clearly silhouetted against the scene, but the smokestacks, harbor docks, and all other cityscape views are distorted by a combination of smoggy air quality and dim morning sunlight. This was how Monet actually saw the landscape. The objects in the distance appeared hazy and blue; the water looked a sickly green color, not deep blue. The people on the boats were indistinguishable, and the sun was the whole scene's only clear object. Obviously Monet knew that there were smokestacks and a harbor and a town there; it wasn't like he was oblivious to the subject matter he was painting. But he painted exactly what his eye saw, not what he knew to be there. He knew quite well what a boat looked like up close; but if this was how it looked from a distance, then he had to paint it this way. Paul Cézanne once said of the artist, "Monet is only an eye, but what an eye!"