The phrase "de novo," as it pertains to legal proceedings, is used in more than one context. You will encounter the phrase "de novo review" and "trial de novo." De novo review generally refers to a "standard of review" that an appellate court will apply when determining the correctness or error of a lower court. Under a de novo standard of review, in theory, the appellate court reviews the facts and legal argument before it as if the lower court had not rendered its decision in the first instance. The trial court is entitled to no deference under this type of review.
A "trial de novo" is often used to refer to a procedure whereby a litigant may "appeal" from an administrative decision to the lowest level of the local courts, typically referred to as the Superior Court. Consider this example - a litigant is owed overtime wages by an employer and files a complaint with his/her local labor enforcement agency (an administrative proceeding). That proceeding does not go well, so the employee decides to appeal his or her case. Instead of going to a Court of Appeal, the employee's case would be "appealed" to the local Superior Court, where the trial judge would conduct a "trial de novo," reviewing anew all of the facts and evidence as if the labor board had not been involved in the first instance.
No, de novo is not a new trial, it is a hearing or review. It is different from appellate review in the sense that appellate courts only examine the transcript of the trial to see if an error in law has occured. Did the Judge or attorneys make some fatal error which would alter the outcome of the case.
De novo review may decide the initial question of the case as though they were the original body to hear the case. A de novo court is not bound by the decisions made by a previous court. Appellate courts are bound by the ruling that the previous court rendered. Appellate courts do not decide guilt or innocence, they decide if the judge or lawyers made a mistake while trying the case.