What is InfoPath?

InfoPath is an application of Microsoft to create XML based Data entry Forms and targeted for end users and developers. The flexibility to create more sophisticated forms with little or no code is one of the highlighted advantages of using InfoPath.
InfoPath Designer is the tool to create Forms and it has a ribbon interface plus the Designer tools. InfoPath is a part of Microsoft Office Professional Plus.

What are the advantages of using InfoPath with SharePoint?

You might be thinking that SharePoint provides Lists which creates Forms for data entry. Then why we need InfoPath Forms?
The answer would be additional features provided by InfoPath like:

• Rules support for validating Fields
• More flexibility in capturing repeated entries
• Managed Code Support
• Digital Signature support
• Users without Programming skills can create Forms