Blocks are modular and reusable pieces of data or content. A block has primary user (indicated by user_id) and can only be edited by the user who created it. However, any block can be reused in multiple channels (this is called a connection). The channels a block appears in across Arena are listed in the blocks' connections attribute.
These connections are shown depending on the authenticated users access. For example, if a block appears in 5 public channels and 2 private channels, an unauthenticated request will display only the public channels a block appears in, but an authenticated request will show the public channels as well as the private channels that the authenticated user has access to.
Blocks can also take many forms: a text, a link (a captioned link to an external website, with a screenshot), an embed (embeddable media such as a YouTube or Vimeo video), an image (either uploaded from a users' computer or saved from an external website), or an attachment (a file uploaded from a users' computer or from an external website, see also: What file types can I upload to Arena?).