The two tags serve different purposes. The <input> tag defines the button you see (it's appearence in value) and what it does (it is a submit button defined by type="submit"). Essentually it is a data input point.
We've inclued the name attribute for completeness as in larger forms it defines a value to the selection of this button (here "button")
Note there is no </input>
The form tage defines what the form does with the data. The method attribute defines how it is sent on (more on this later) whilst the action attribute defines where it is sent too. In this form's case the submission triggers the form to send info and get/retrieve the webpage www.fluffbucket.com/. We've added a target attribute to make it open in another window
Many forms are quite complex and you can have numerous inputs within one form. You can subject inputs to most text alignment rules as well (you can even put them into tables)
Have a look at this form. Both buttons will open our home page
<form method="get" action="http://www.fluffbucket.com/" target="_blank"> <input type="submit" name="button" value="I can open the home page"> <input type="submit" name="button2" value="So Can I"> </form>