A channel, or board, is a group of discussions that all fall under a single shared topic. Like the Team Fortress 2 channel talks about Team Fortress 2 game or server discussions.
A channel has 4 possible statuses.
Tags are as they sound, some channels allow for additional tags to aid in searching for a discussion later on. Tags aren't custom, they can only be selected from a list of tags that a particular channel allows.
Additions, or templates, or extra bits/tools that can be added to a discussion when the need arrives. An example of this is is this particular discussion, that has the thread_sidebar and table_of_contents flags enabled. This allows this discussion to have a table of contents for easier reading.
Additions are only accessible to admins with the proper permissions, eventually, more members will get access to these flags!
Discussion overrides or Channel overrides refer to when the normal layout of the respective page has been replaced with custom content that would normally require extra effort or cannot be done with markdown.
Basically, it can be used to make a discussion that has custom code inside of it.
A protected discussion or post is one that has been previously reported at some point in the past, determined not to be breaking any of the SCG rules, and marked as protected. A protected discussion cannot be reported as it was found to not be breaking the rules.
If the protected discussion or post is edited at all, by an admin or the topic's owner, it will lose its protected status.