Posting an article can take two forms. The first is to create original content. The second is to user the URL of an item to import the content.
Both forms of Post are created from the same starting point. Select a topic and then select "Post to topic" on the right side of the topic header. A form will be presented. From this form you can:
create original content. Note that a URL can be assigned to the post if you would like it to resolve to a given address.
insert a URL for content and it will be imported if possible.
The second method can be very useful for importing a sample item and finding similar items based on tags or the recommendation below the article.