Definition
Tool tips or hints are messages that are triggered automatically when the cursor hovers or clicks on an image, icon, input field, hyperlink or some other asset on a graphical user interface. For conversion tool tips can be particularly important for assisting users in completing forms, including registration and check-out forms.

Source: LeoVegas.com
In forms tool tips remind users what is required (e.g. number of characters, a mix of numbers and letters). This is an important role of tool tips for conversion. People often want reassurance that information won’t be misused. If users are not convinced that a piece of information is not needed they are more likely to skip the field.
They also confirm the nature of hyperlinks, icons and images to give instant feedback when a user’s cursor hovers or clicks on the item.
Resources:
Conversion marketing – Glossary of Conversion Marketing.
Over 300 tools reviewed – Digital Marketing Toolbox.
A/B testing software – Which A/B testing tools should you choose?
Types of A/B tests – How to optimise your website’s performance using A/B testing.










