Users by Traffic Type
In Google Analytics a unique visitor is considered a New User.
The internet actions the user takes determines the traffic type.
Direct
URL-typed visits
Visits from:
Bookmarks
Email signature. If someone clicks on a website link in a email signature in Outlook, no referrer will be passed to the user browser and this visit will be attributed to direct.
Untagged email campaign. When someone clicks on an untagged link (link which does not include a Google Analytics tag) from in their email client (Outlook, Lotus Notes or Thunderbird), no referrer will be passed and the visit will be classified as direct. This is actually one of the most common cause of spikes in direct traffic. It is therefore very important to correctly tag email campaigns.
Note: This only applies to email clients. If someone clicks on a link from a Webmail (Hotmail, Gmail, Yahoo), the visit will be classified in Referring traffic.
Shortened URLs in Facebook, Twitter or Social Media. The shortened URLs do not pass referrers, so all your traffic coming from your shortened Twitter updates will be classified as Direct. To ensure it is classified in the right bucket, make sure you tag your URLs.
A Document (Word, Excel)
Secured environment or intranet, where the Firewall or Proxy strips the referrer leading to the referrer not being passed.
Untagged Mobile Ads link will be classified as Direct.
Page of your website missing the Google Analytics tag. If someone clicks on a link from a page of your website,which does not have the Google Analytics tracking code, the visit will be reported as direct. It is therefore quite important to ensure that GA tracking code is added any new page you might create.
A link on a HTTPS page pointing to a HTTP page or on a HTTP page pointing to a HTTPS page. For security reason, the referrer is not passed on this type of links and therefore the visit will be categorised as direct. A quick fix is to ensure that your URL are tagged correctly.
Someone, who has modified its browser configuration to avoid passing the referrer ( not so common).
Links encoded in Javascript. For some browsers (IE), links encoded in Javascript will not pass the referrer and therefore be classified as direct traffic. Some display networks use this type of links, so it is advised to always use tagged URLs to avoid bad surprises.
Unproperly coded Redirects. 301 Permanent Redirects usually pass the referrer information so your traffic will be classified correctly, however in some cases, 302 Temporary redirects can lead to visits being classified as direct traffic.
Note: Quite logically, untagged paid search campaigns usually do not end up in Direct traffic but in the search engine bucket. it is however important to tag paid search campaign correctly
Referral
Referral traffic is used to describe visitors to your site that come from direct links on other websites rather than directly or from searches.
For example, other sites that like what you have to say or sell may post a link recommending your site.
You can also try to drive your own referral traffic by leaving links on other blogs or forums you have joined. Pay-per-click ads also count as referral traffic.
Organic
Organic search results are listings on search engine results pages that appear because of their relevance to the search terms, as opposed to their being advertisements. In contrast, non-organic search results may include pay per click advertising (Wikipedia).