Are email campaigns sent to users or contacts?

Email campaigns are generally sent to users on an account, not contacts, however, this will depend on how you start your campaign. To send a campaign to users on accounts in bulk, navigate to Partner Center > Marketing > Campaigns > 3 dots next to desired campaign > Add account list to campaign. 

OR 

To send a campaign to users on one specific account, navigate to Partner Center > Accounts > Manage Accounts > Select specific account > Click the 3 dots next to the desired user > Campaigns. 

You can also send campaigns to contacts via Partner Center > Contacts > Contacts > Select specific contacts > Actions > Add to Campaign.

You can also send campaigns to contacts via Sales & Success Center > Accounts > Select account > Actions > Start email campaign.