There are two ways Users* can send an email to contacts with a particular contact tag:

Send via Quick Send

Start by navigating to the Emails menu. Create a new email, or click on an email from your saved list.
From here:

• Once in the email editor, select Quick Send (if you have just created the email you will need to Save first to enable the Quick Send button).

• Within the Choose recipients module, the Search for Contacts, Tags & Audiences option is the default.
• Start typing in the tag you would like to send to.
• Select the correct tag from the list of matches in the dropdown.
• Add more tags if you wish by repeating this process.
• In the Date & Time module, select when you would like to send, e.g: Send Immediately
• Select Send

Send via Campaigns

Start by navigating to the Campaigns menu. Create a new campaign.
Once in the campaign builder:

• Within the Choose recipients module, the Search for Contacts, Tags & Audiences option is the default.
• Start typing in the tag you would like to send to.
• Select the correct tag from the list of matches in the dropdown.
• Add more tags if you wish by repeating this process.
• In the Date & Time module, select when you would like to send, e.g: Send Immediately
• Click Send

Send to more than one type of contact

If you would like to send to more than one type of contact - e.g: a tag and an individual contact, just select both:

• Send an email to a single contact
• Send to an audience or audiences 

* Admins have the options to Send to All Contacts, or Audiences