Invoice generators are online tools that allow you to create invoices within your web browser.
They follow a template to ensure you don't forget to include important details when filling out your invoices. They are not like invoicing/time-tracking apps like FreshBooks or Zoho Invoicing because invoice generators are single-purposed: They simply help you create an invoice and do not have any advanced features like time-tracking, financial accounting, and team collaboration.
Think of invoice generators as Internet-enabled alternatives to desktop-software invoice templates that are commonly used in Microsoft Word, Adobe Acrobat or other desktop publishing software.