A content resource is a piece of content that is used as input when performing an activity. For example, it could be an instructional text that instructs the person that shall perform the activity how to do it, or an image that will be part of the final content product.
A content product (sometimes called publication) is one or more content resources that have been put together for a specific audience or user. For example, it could be a newsletter, a web page, a video clip, or an entire web site.
A content asset (sometimes called digital asset) is content that is said to have a certain (usually monetary) value to its the owner, usually a business. Hence, both content resources and content products can be seen as content assets.