What are the possible options for UserType?

Your best bet is to refer to the Salesforce api docs. They do a good job of explaining the fields and their possible values. The link to the doc in question is: https://developer.salesforce.com/docs/atlas.en-us.212.0.api.meta/api/sforce_api_objects_user.htm

I've included the snippet from the UserType field below.

Description The category of user license. Each UserType is associated with one or more UserLicense records. Each UserLicense is associated with one or more profiles. In API version 10.0 and later, valid values include:

  • Standard: user license. This user type also includes Salesforce Platform and Salesforce Platform One user licenses. Label is Standard.
  • PowerPartner: PRM user whose access is limited because he or she is a partner and typically accesses the application through a partner portal. Label is Partner.
  • CSPLitePortal: user whose access is limited because he or she is an organization's customer and accesses the application through aCustomer Portal. Label is High Volume Portal.
  • CustomerSuccess: user whose access is limited because he or she is an organization's customer and accesses the application through a Customer Portal. Label is Customer Portal User.
  • PowerCustomerSuccess: user whose access is limited because he or she is an organization's customer and accesses the application through a Customer Portal. Label is Customer Portal Manager. Users with this license type can view and edit data they directly own or data owned by or shared with users below them in the Customer Portal role hierarchy.
  • CsnOnly: user whose access to the application is limited to Chatter. This user type includes Chatter Free and Chatter moderator users. Label is Chatter Free.

Tim Smith's excellent answer already covers the User Types that are currently documented by Salesforce, so I'll focus on the "undocumented" ones.

This screenshot was taken from a picklist lookup, triggered from the List View customization screen for the User object. It should be a complete list of all User Types as of Spring ’14.

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Comparing this list to the one that Tim found with the UserType field on the User Object API Documentation page, it would appear that there are two "undocumented" User Types.

  • Guest
  • Self Service

As Ryan Guest pointed out, the "Guest" User Type is used exclusively for unauthenticated Force.com and Site.com sites users. This link, provided by Scott, has more information about how this license type is used: Force.com Sites Guest User Licenses.

That leaves the "Self Service" user type.

The "Self Service" User Type appears to have been deprecated, along with the Self-Service Portal, when Spring ’12 was released. Only orgs that existed prior to Spring ’12 continue to have access to the Self-Service Portal. More information about the Self Service Portal can be found in the Self-Service Portal Implementation Guide.


Adding one more...

If the running user is the Salesforce to Salesforce (S2S) Connection User, then (as of V32)

UserInfo.getUserType = 'PartnerNetwork'
UserInfo.getFirstName = 'Connection'
UserInfo.getLastName = 'User'
UserInfo.getName() = 'Connection User'
UserInfo.getUserRoleId() = null
UserInfo.getProfileId() = some profileId that can't be inspected (insufficent privileges)
UserInfo.getUserId() = some userId that can't be inspected (insufficient privileges)

The S2S Connection User can not be queried for in SObject User