There are several icons in the recommended content list. Here is a guide that defines them.
You will typically see the following images to the right of the content title in the list view:
From Left to Right these are:
Start a Discussion in Chatter - The speech bubble icon allows you to comment on an asset for others to see within Salesforce.
Download - The cloud with an arrow icon allows you to download the content for use offline.
Preview - The screen icon will take you to the preview of the content to make sure it is exactly what you want to share with your prospect.
Share - The envelope icon directs you to a dialogue to share the content with a prospect.
You may also see the following to the right of the content title:
Join a Discussion in Chatter - Much like the speech bubble above, this allows you to comment on an asset. This means that someone else has already commented and you can join the discussion to see what has been said about the content.
Weblink Preview - If a piece of content is a web link versus a document such as a pdf or xls, you will see the arrow in a box icon to indicate that it will open in a new tab in your browser instead of in the in-line preview box.
The icons to the left of the content title denote what kind of file the content is.