Lincoln repeats certain key words for emphasis. "We have come to dedicate a portion of that field. . ." "But, in a larger sense, we cannot dedicate. . ." "It is for us, the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced."
Read the speech for repetition of words "consecrate," and "devotion."
Parallelism has to do with sentence structure. It is the use of the same grammatical form in a series of two or more ideas to call attention to those ideas, to create a balance in the sentence, and to create a pleasing rhythm to the sound of the sentences.
"and that government of the people, by the people, for the people, shall not perish from the earth."
In this case, the repeated grammatical element in the series of three phrases is a prepositional phrase because each phrase begins with a preposition, "of," "by," and "for" followed by the object of the preposition "the people". Certainly the effect would have been different had he said "and that a democracy shall not perish form the earth."
Take a look at parallelism here:
"The world will little note nor long remember what we say here, but it can never forget what they did here."
There is a nice balance in this two part sentence with almost identical two pieces "what we say here" and "what they did here." This is parallel structure because the same exact word order is repeated in a series of two ideas, and thanks to doing that, Lincoln establishes the desired effect--to contrast words with actions, and to also leave the most important idea for the end of the sentence, thus leaving a more lasting impression on the audience.
There are some powerful rhetorical speaking devices that Lincoln uses in his speech. One such device is the use of repetition that is employed to punctuate the purpose of the end of clauses or sentences. This can be seen in his concluding thought to the address: "that government of the people, by the people, and for the people shall not perish from the Earth." The repetition of preposition followed by "the people" punctuates the end of the sentence with the repeating of the components or a democratic republic. Another tool employed in the speech is the use of antithesis, where one idea is contrasted with its opposite force in the same sentence or clause. In bringing out its contrast, the antithesis illuminates the purpose or meaning of the thesis. This is seen in the phrase, "The world will little note, nor long remember what we say here, but it can never forget what they did here. It is for us, the living, rather, to be dedicated her to the unfinished work which they who fought here have thus so far nobly advanced." Contrasting the meaning of the actions of the soldiers to the lightness of words brings to light the notion of sacrifice. It also creates the image of motivating those who survive to commit themselves to a goal of those who sacrificed their lives. In using the antithesis of death as survival, Lincoln is able to create focus and purpose within the hearts and minds of those who stood in support of the Union and its soldiers.
It is repetition and parallel construction.
Look at page 146 of your american lit study guide.