Abstraction usually refers to writing a part from the whole article or work, rather than completely copying the whole content. Excerpts are usually representative or key paragraphs, sentences or words to highlight their importance or aesthetic feeling.
Abstract refers to the omission or abbreviation of some parts in order to simplify the writing process or highlight the key points when copying the whole article or work. Excerpts are usually irrelevant or repetitive parts, or abbreviated to suit a particular writing format or layout.