If you want an item to be publicly available, share it with everybody. This means that anybody who visits ArcGIS Online, including people who are not signed in, can find and use your item.
You can keep your item semi-private by sharing it with one or more groups you belong to. This keeps it private except to group members. Alternatively, you can share an item with everybody and a group when you want everybody to have access to your item, but you also want to highlight the content within a group setting. This is especially appropriate for focused group work where all members benefit from seeing a list of specific content they can use for collaboration and exchange. For example, as a fire agency that produces burn maps, you want the general public to find and view the maps but you also want members in the fire agency group to use the maps as templates for creating their own local versions.
Another way to share an item is by distributing the URL to its properties page, for example in an email to a co-worker. Only those with permission to access the item will be able to open the URL.
To share an item with everyone, you must be signed in to your account:

NOTE: You can also share an item when you are editing its content. See Editing an item for details.
To share an item with a group, you must be signed in to your account:

NOTE: You can also share an item when you are editing its content. See Editing an item for details.