apex:form, it is a section of visualforce page that allows users to enter input and then submit it with command button or link.

It is a best practice to use only one apex:form tag in a page or custom component.


<apex:page standardController="Account">   

    <h1>Update Account Details</h1>   


        <apex:inputField value="{! Account.Name }"/>       

        <apex:commandButton action="{! save }" value="Save" />   



apex:form attributes

accept, acceptcharset, dir, enctype, forceSSL, id, lang, onClick, ondblclick, onkeydown, onkeypress,  onkeyup, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup, onreset, onsubmit, prependId, rendered, style, styleClass, target, title.

Refer this for more details https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_form.htm