Officially known as "DD Form 214, Certificate of Release or Discharge from Active Duty", the DD 214 is the single most important piece of paper one can obtain while serving in the US military. It's sometimes called a "Golden Ticket".
You receive a DD 214 upon separation or retirement from the military, and it details all significant information from your time in service. Breakdowns of your total combined service time, all qualifications and decorations you may have earned, overseas time, combat deployments, promotions, specialties, and a record of training and schools. It is a complete and verified record of your entire military career.
There are several versions of the DD 214, most of which are identical save in name and destination upon out-processing, notably the Service 2 and Member 4 copies. The first (original) copy presented to the member will be the Member 4 copy, and all subsequent copies (due to lost or damaged Member 4 copy) will be certified copies of the Service 2 copy. They are identical and contain the same information.
Other various copies are retained in the service's records (Service 7, Service 8), sent to the Department of Veterans' Affairs (Veterans Affairs 3), and consumed or destroyed in the bureaucracy involved in creating and distributing the other copies.
There are two types of DD 214, "short" and "long", with the "long" by far being the more common and useful. Sometimes known as "edited" (short) and "unedited" (long), the extra information in the long-form are most often used when dealing with veterans' organizations, employers, and further dealings with the federal government.
It is important, if niche, information to note that the DD 214 is both complete and also unclassified; there is no such thing as a "redacted" or "classified" DD 214, and there are absolutely no qualifications, schools, types of training, or overseas deployments that are not listed on a DD 214. While the "short" form may not have certain information on it, it is certainly not due to the secrecy of that information.
Anybody claiming to have had classified medals, deployments, or training either not shown on the DD 214, or blanked out (redacted) is a liar and a fraud. This type of scam is most often encountered when dealing with someone who is pretending to have been Ranger, Special Forces or SEAL qualified, or claiming to have received high military honors during a classified operation. The former is unambiguously reflected in one's service and training records and therefore on the DD 214, and the latter would have been awarded with a two-part citation, one part unclassified and kept in the service record, and the other part below a "tear-line" that is kept secret.