As commented by jmfsg, the bulk of visitors arrive from search engines and other sites. There are, however, no publicly visible statistics that show what actions these users take. It's fair to state from my own observations that the bulk of visitors are not the people who vote or answer questions on the site though - sure there will be exceptions to this, but in general.
A small subset of regulars will notice that we have a feed in The Bridge that lists all new questions shortly after they come in - but this is as stated just a small subset of our active users.
The bulk of users will work through the site activity page which shows all currently active content on the site regardless of age. This is one of the reasons why one user flooding the site with content (whether new questions, new answers, edits, or other activities that result in a question being bumped onto the activity page) is bad, since it pushes content off this page, in addition to giving the illusion that the site has less active users that would otherwise be apparent from the information on this screen.
Other than the site activity page, there is the new questions page which shows all new questions on the site, in the order they were asked (newest to oldest).
The important thing to consider is, it's easy to find a specific question using a search engine when you're looking for something specific, however it's usually our core active userbase that is responsible to providing the actual answer to a question asked on the site, and these users will find questions to answer using the site activity page, the new questions page, and any the tag pages of any tags that they're monitoring/interested in.