OnClick -> Used for both Asp.Net button i.e. command button, link button and HTML button.

OnClientClick -> Used for only Asp.Net button.

For Example :-

<script type = "text/javascript" language="javascript">
function get_alert()
{
alert("hello");
}
</script>

To understand this,

<asp:button id="btn_show_message1" runat="server" OnClientClick = "get_alert(); return false;" Text = "Show message using OnClientClick"/>

<input type="button" id="btn_show_message2" name="button" value ="Show message using OnClick" onclick = "get_alert();"/>