The purpose of the collection feature is to allow you to gather selected images in one place. Images from the search results, your own as well as images from other contributors can easily be placed in your collections.
All collections get an auto-generated URL address that you can send to friends or customers. People you send a collection to can only view it and are not able to edit it in any way. You on the other hand, are able to rename, organize and move images between your different collections.
Your collections are not visible on your portfolio page, hence only those with the correct URL address are able to view it.