Normal HTML(5) forms work this way, but if your form is mostly managed through JS you don't necessarily get this default functionality.
If you don't actually use the <form> tag but instead just have an input field and a submit button, pressing Enter will do nothing. In that case there are two solutions:
1) Put the input fields inside a <form>. (If you need JS to control form submission, you can always include an onsubmit event handler that returns true or false depending on whether the user filled in the form correctly). Example: http://jsfiddle.net/LFkGZ/
2) Attach a keydown event to <body> so that when the user presses Enter you can submit the form. Example: http://jsfiddle.net/6r7gh/
