What's the difference between <%# %> and <%= %>?

<asp:SqlDataSource ... Runat="server"
 ConnectionString="<%$ ConnectionStrings:Northwind %>"
/>

<asp:Literal Runat="server"
  Text="<%$ Resources:MyResources, MyText %>"
/>

var e = document.getElementById('<%= lblDescription.ClientID %>');
e.innerHTML = 'getElementById('lblDescription') will be null';