Web forms are subscription forms you embed on your website to capture leads and add them straight into a contact list. You manage them under Setup → Web Forms, starting either from a pre-designed template (subscriber forms, contact forms, minimal designs) or a blank one.
Creating a form is a 4-step wizard:
Form Details — name the form, choose the contact list, pick the sending nodes used for its emails, and select which custom fields appear (Email is always included; drag fields to reorder them). Here you also enable double opt-in ("Require email confirmation"), a thank-you email, Google reCAPTCHA (V2 or V3) spam protection, admin notifications on signup, and duplicate handling (Skip or Update existing contacts).
Email Confirmation — the confirmation page or redirect, plus the confirmation email content, which must include the %%confirmurl%% confirmation link.
Welcome Email — the thank-you page or redirect and the welcome email content.
Error Handling — what visitors see if subscription fails, using the %%error%% variable.
With double opt-in, contacts are added immediately with Unconfirmed status and only become Confirmed (and eligible for campaigns) after clicking the confirmation link; without it, they're Confirmed right away. After saving, use Get HTML to copy the embed code for your site, or View, Download, Copy, and Copy Post URL from the row menu. Worth knowing: the contact list must have a sending domain configured, and at least one sending node must be selected.