On the Internet you may find a great variety of different content - photos, sounds, animations.. To be able to use that stuff legally, you have to get the appropriate license from the author/content owner. Stock content is usually referred to as content licensed for use in a set of cases - web usage, editorial, print, etc.
Free stock content
We mark websites as having "Free stock content" if they have at least 1-2 thousand photos
(hundreds - for vectors, not less than a hundred - for templates) of items licensed for a specific use and free for download immediately of after a registration.
Royalty-free content licensing
That type of licensing usually means the use of content a multiple number of times by more than one person/organisation. In general, that type of content is cheap, it may be downloaded for a small fee.
Rights-managed content licensing
Also referred to as "sell-the-rights" or "exclusive rights" licensing. In most cases, the buyer acquires the content item for a considerable sum (tens or hundreds times higher, than with royalty-free license) and receives the exclusive rights to use it. The content author retains the authorship of the content (the content buyer can not pretend to be its creator).
If the website is marked as "accepting contributors", it means that you may send them your own content and sell/distribute it according to the website terms.
If the website is marked as "accepting affiliates", it means that you may participate in a referral program on the website - usually get money or other bonuses for bringing new visitors/buyers/contributors to the website.
The content selling sites may use different models to sell content - "one at a time", subscription, buying internal website "credits", etc.
Means of payment
The ways, you may pay money to the content sellers - credit card, Paypal, Moneybookers, cheques, and many others.
That is almost clear - if the content seller specialises on a specific content type, we'd like to note that.
Content resources found on the Internet greatly vary by "weight category" - from huge photo agencies to small one-person-supported websites and albums. We meet a large number of these "small" class resources which are of no real use to designers and are not strictly specialised. They still may be useful for someone who is desperate for finding a rare photo/video/audio content and just would like to look just everywhere for it. Numbers vary by type of content - if a photo
resource with 10 thousand images is considered relatively small, the same number of web templates
of good quality may be classified as "medium" to "big".