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A web form is a user interface for collecting data to be submitted to a server. It typically appears as a form on a web page and uses HTML form controls to collect inputs such as text, numbers, dates, selections, and files. The data is sent to a server for processing via HTTP methods, usually POST or GET.

Core structure includes a form element with action and method attributes, input controls like input, textarea, select, and button; labels associated with controls; and sometimes fieldsets and legends to group related controls. Good forms use semantic markup and clear instructions.

Validation ensures data integrity. Client-side validation uses HTML5 attributes (required, type, pattern) and JavaScript; server-side validation is essential. Data may be sanitized and stored or processed; error messages guide users.

Security and privacy considerations: forms can be vectors for attacks (CSRF, XSS); mitigations include CSRF tokens, input sanitization, HTTPS, same-origin policies, proper handling of file uploads, minimal data collection, and consent notices.

Accessibility and usability and common types: forms should be navigable by keyboard, labeled properly; error messages accessible; common types include contact forms, login/registration, checkout, survey; trend toward multi-step forms, responsive design, progressive enhancement, and client-side frameworks.