Forms allow users to enter data into a website. They contain form elements like text fields, drop-down menus, and buttons. The <form> element defines a form, while <input>, <textarea>, <select>, and <button> elements create specific form controls. Forms submit data via GET or POST requests, and attributes like action, method, and target control submission. Common elements include single-line text, passwords, textareas, checkboxes, radio buttons, drop-downs, file uploads, hidden fields, and submit/reset buttons.