Labels or instructions are provided when content requires user input. (Level
A)

Sufficient Techniques for 3.3.2 – Labels or Instructions

• 1.G131: Providing descriptive labels AND one of the following:

• G89: Providing expected data format and example
• G184: Providing text instructions at the beginning of a form or
  set of fields that describes the necessary input
• G162: Positioning labels to maximize predictability of relationships
• G83: Providing text descriptions to identify required fields that
  were not completed
• 2.H44: Using label elements to associate text labels with form controls
  (HTML)
• 3.H71: Providing a description for groups of form controls using fieldset
  and legend elements
  (HTML)
• 4.H65: Using the title attribute to identify form controls when the label
  element cannot be used
  (HTML)
• 5.G167: Using an adjacent button to label the purpose of a field

Note: The techniques at the end of the above list should be considered “last
resort” and only used when the other techniques cannot be applied to the
page.
The earlier techniques are preferred because they increase accessibility to
a wider user group.

Advisory Techniques for 3.3.2 – Labels or Instructions

• G13: Describing what will happen before a change to a form control
  that causes a change of context to occur is made
• ARIA1: Using Accessible Rich Internet Application describedby property
  to provide a descriptive, programmatically determined label
  (ARIA)
• ARIA4: Using Accessible Rich Internet Applications to programmatically
  identify form fields as required
  (ARIA)
• Providing linear form design and grouping similar items

Failures for SC 3.3.2 – Labels or Instructions

• F82: Failure of Success Criterion 3.3.2 by visually formatting
  a set of phone number fields but not including a text label

http://www.w3.org/TR/UNDERSTANDING-WCAG20/minimize-error-cues.html