The recent activity page is where you can see what has been created. You may filter the activity page by type and/or search by keywords.
Posts are the primary items of content on the site. You may post to conversations, which allow for posts to be organized.
If you have something to add to what has been posted, use a reply. This allows for related material to be grouped together.
On some sites, people follow. Here, we listen. When you listen to a person, you will be notified (see the notifications page) when that person creates something. When you listen to a creation, you will be notified when someone posts or replies to that.
The notifiations page shows activity that has occurred that you are listening to. Click the red x to hide a notification. In this way you can work through your queue.
By default, you will receive daily email digest notifications so that you don't need to visit the site every day to see what's happening. Daily digest notifications may be turned off under account settings.
Conversations have stewards, who keep an eye on the health the conversation. The creator of a conversation is automatically made its steward, and may add stewards as desired.
You may send messages to people on the site. Look for "Send Message" links on profile pages. You may also change your message settings under account settings.