What's the difference between these 2 journals? Oceanography is peer-reviewed, whereas National Geographic is a popular-press title.
Peer review is scientists' and other scholars' best effort to publish accurate information. Each article has been submitted by a researcher, and then reviewed by other scholars in the same field to ensure that it is sound science. What they are looking for is that:
Peer-reviewed journals are generally considered "primary source" material: When a new scientific discovery is made, a peer-reviewed journal is often--but not always--the first place it appears.
Popular and trade publications are not peer-reviewed, they are simply edited. That does not mean they are any less potentially truthful or informative--most popular and trade publications take pride in careful fact-checking.* But when the topic is scientific research, the information is generally "secondary": It has already appeared elsewhere (usually in a peer-reviewed journal) and has now been "digested" for a broader audience.
Peer-reviewed journals will always identify themselves as such. If you want to verify that a journal is peer-reviewed, check Ulrich's Periodical Directory.
Some sources of peer-reviewed articles: