From Wikipedia: In grammar, parallelism is a balance of two or more similar words, phrases, or clauses. The application of parallelism in sentence construction improves writing style clearness and readability. Parallelism may also be known as parallel structure or parallel construction. In English, parallelism of the predicate provides for one of the few structural situations in which the subject for each verb does not need restatement. Parallelism is often achieved in conjunction with other stylistic principles, such as antithesis, anaphora, asyndeton, climax, epistrophe, and symploce.[1] Parallelism strengthens sequences described. [edit] Examples Some examples of parallelistic sentences: Wrong: The investigators added drug A, added drug B, and measured the pH. Correct: The investigators added drug A and drug B, then measured the pH. Wrong: You must take particular care in the preparation of your materials and methods, your bibliography, and your index. Correct: You must take particular care in the preparation of your materials and methods, of your bibliography, and of your index. Wrong: Please leave your name, your number, and a message should also be left. Correct: Please leave your name, your number, and your message.

Parallelism is such an abstract idea I wouldn't bother too much about it. Just concentrate on making nice sentences, and the understanding will follow.