What Is an Invoice? Definition, Types & Required Fields (2026)

Quick answer: An invoice is a document a seller sends to a buyer that requests payment for goods or services already delivered or agreed upon. It lists who is billing, who is being billed, what was provided, how much is owed, when payment is due, and how to pay.

CatInvoice is a free browser-based invoice generator (no signup) that helps freelancers and small businesses create professional PDF invoices with templates, automatic totals, and local save — useful once you understand what belongs on the document itself.

What is an invoice? (Definition)

An invoice is a commercial billing document. It is not a contract by itself, and it is not proof that payment was received. Its job is to request payment with enough detail that the buyer's accounts payable team can approve and pay it without back-and-forth.

In one sentence: An invoice tells your client what you delivered, how much they owe, when it is due, and where to send the money.

For authoritative context on business payments, see the U.S. Small Business Administration guide to getting paid and the Investopedia invoice definition.

Invoice vs quote vs receipt: what is the difference?

These documents are often confused. Each has a different purpose in the sales and payment cycle.

If you need help writing payment deadlines on the invoice itself, read our invoice payment terms guide.

Common types of invoices

Most businesses use a few standard formats depending on how they bill.

• Standard invoice: One-time bill for completed work or a product sale.
• Recurring invoice: Sent on a schedule (monthly retainer, subscription, or maintenance).
• Pro forma invoice: Estimated bill before final delivery; often used in international trade.
• Interim / progress invoice: Partial bill tied to project milestones.
• Final invoice: Last bill that closes out a project, sometimes net of deposits.
• Credit note: Adjusts or reverses a previous invoice (refund or correction).

What must be on an invoice? (Required fields checklist)

Requirements vary by country and industry, but professional invoices almost always include the following.

For a full walkthrough of building each section, see how to create an invoice.

When should you send an invoice?

Send the invoice as soon as the trigger event happens — do not wait weeks after delivery.

1. After one-time delivery: When the product ships or the service is completed.
2. Per milestone: When a contract phase is signed off (design approved, build deployed, etc.).
3. On a schedule: The same day each month for retainers (e.g., 1st of the month).
4. After a deposit: Send a deposit invoice before work; send the balance on completion.

Late invoicing is one of the fastest ways to delay cash flow. Our guide to getting paid faster covers follow-up timing and polite reminders.

How to create an invoice (simple workflow)

You do not need full accounting software to issue a valid invoice. Many freelancers use a focused invoice tool instead.

1. Open CatInvoice in your browser — no account required.
2. Enter your business details and pick a client (saved locally for next time).
3. Add line items with clear descriptions, quantities, and rates.
4. Set tax, discount, currency, and payment terms in the sidebar.
5. Choose a template, preview the layout, and download a print-ready PDF.

CatInvoice stores drafts and saved invoices in your browser, supports 60+ currencies, and calculates totals automatically. It does not send email or sync to the cloud — you attach the PDF in your own email app.

Are invoices legally required? (Tax & record keeping)

Rules depend on your jurisdiction. In many countries, businesses must issue invoices for B2B sales and keep copies for tax audits. Self-employed workers in the U.S. should follow IRS self-employed tax guidance and retain income records that match invoices and payments.

For filing systems and retention habits, see how to organize and track invoices.

Common invoice mistakes

• Missing a unique invoice number or due date
• Vague line items (e.g., "consulting" with no scope)
• Billing the wrong legal entity or contact
• Forgetting tax ID or registration numbers where required
• Sending editable Word files instead of a PDF

Tip: Treat every invoice like a payment instruction, not a marketing flyer — clarity beats decoration.

Frequently asked questions about invoices

What is an invoice in simple terms?

An invoice is a bill you send to a customer listing what you provided, how much they owe, when payment is due, and how to pay. It is sent before or until payment is completed.

Is an invoice the same as a bill?

In everyday language, yes — both request payment. In accounting, "invoice" often refers to money you are owed (accounts receivable), while "bill" may refer to money you owe a supplier (accounts payable).

Can I create an invoice without accounting software?

Yes. A dedicated free invoice generator like CatInvoice is enough for creating, saving, and exporting PDF invoices without QuickBooks-level complexity.

What is the difference between an invoice and a receipt?

An invoice asks for payment. A receipt proves payment already happened. Send the invoice first; send a receipt after the client pays.

What payment terms should I put on an invoice?

Common terms include Due on receipt, Net 7, Net 15, and Net 30. Pick terms that match your cash-flow needs and your client's accounts payable cycle. See our payment terms guide for examples.

Does CatInvoice store my invoices in the cloud?

No. CatInvoice keeps drafts and saved invoices in your browser storage on your device. Export PDFs or JSON backups if you need copies elsewhere.

Key facts (at a glance)

• Definition: An invoice is a payment request document sent by a seller to a buyer.
• Not a receipt: Invoices request payment; receipts confirm payment.
• Core fields: Parties, invoice number, dates, line items, totals, payment instructions.
• Best practice: Send promptly after delivery and use a clear PDF format.
• CatInvoice: Free, browser-based invoice maker with templates and PDF export — create an invoice.