Content protection and content security are closely related topics. Content protection is preventing availability or use of content by those not authorized. Content security is preventing any user (authorized or not) from being able to alter content without detection.
There are many ways to protect content. Typically encryption is used, so that only the intended user(s) may decrypt and use the content. This has advantages. Today’s encryption systems are good, reliable and fast. But key management and distribution is rather less so, since human beings are not very good at security, particularly when they do not understand how it works or they see it as an inconvenience, or do not mind giving their credentials to others (much as they let other people use their ATM cards).
Although this protects the digital content whilst it is winging its merry way around the Internet, it does not protect content from the authorized user, and this is not a good thing. The authorized user can choose if they want to continue the content protection or not. And this may not fit in with the wishes of the content owner.
Of course, continuing content protection can be achieved by using DRM mechanisms rather than just encryption. DRM allows the owner to protect content after it has arrived at an authorized user, so the content protection survives, rather than being replaced by whatever the recipient (a hacker perhaps) arbitrarily decides.
Methods to secure digital content generally use cryptography, usually involving a digital signature. The method is to choose the content to protect and then run something called a one-way digest (a mathematical algorithm) that creates a numeric value unique to that content. To check that the content has not changed you run the digest again and compare the results. If they are the same you consider that the document is authentic in that it comes from the identity claimed and has not been altered. Unfortunately, during 2011 it came to light that fake identities had both been created and stolen, allowing serious hackers to impersonate valid organizations, and again, the question of potential weaknesses in key management and distribution have created problems for mechanisms relying on these identities.
DRM also provides methods to secure digital content. Generally secure content keys are managed because the DRM does its own key creation and distribution, and does not rely on third parties, so it does not fail to secure content because the third parties fail. There are DRM methods using PKI but they pass on the difficulties of management to the unskilled publisher, which is not very helpful since publishers do not think they are security specialists (and security specialists are very quiet).
So it turns out that content security and content protection can be achieved in a number of ways, but unless that is done using DRM technology you will not secure content or protect content in ways that are actually valuable.