Visible to Public
When creating a campaign using the Campaigns page, you need to make sure that you check a box that says "Visible to public?" If this is not checked, the campaign will still exist, but your online donors will not be able view the campaign when submitting donations.
Naturally, you may have campaigns that are not meant to be visible to the public (specific campaigns that are just targeted at certain constituents who won't be online).
Parent & Child Campaigns
Note that if a parent campaign has one or more child campaigns which are publicly visible, then the parent campaign is automatically made publicly visible.
It is not possible to have a "hidden" parent campaign with one or more "visible" children campaigns.