Hudson River School artists were the first really important “school” of artists in the U.S. A school is just a group of artists painting in a similar way (not in a little school house). These artists, including Cole, Church, Bierstadt, Durand and many others, emphasized the beauty of our landscape (vs. the “history” and mythology paintings of Europe) and gave our land a spiritual quality similar to the religious paintings of Europe.
Thomas Cole Expulsion from Eden about 1840 and Albert Bierstadt Yosemite Valley about 1860 or so.
Abstraction is taking any image a step or two away from "reality" by emphasizing something the artist wants to deal with: super bright colors, distorted forms, etc. Sometimes it is mistaken with non-objective art, which has no basis in reality at all.
abstraction by Georgia O’Keeffe Jack in the Pulpit IV and non-objective art by Mondrian both about 1920-30