The AutoPostBack = true permits control to post back to server. It is closely associated with an Event.
<asp:DropDownList id="id" runat="server" AutoPostBack="true" OnSelectIndexChanged="..."/>
This aspx page with the above mentioned drop down list does not need an asp:button to do the post back. When you change an option in the drop down list then the page gets posted back to the server.
Note : Default value of AutoPostBack on control is false.