What is the difference between client-side and server-side validations?

Client-side validations take place at the client side with the help of JavaScript and jQuery etc before the Web page is sent to the server whereas server-side validations take place at the server end and done using programming languages like C#.Net,VB.Net etc.
Client-side validation is faster than server-side because it takes place on client side (on browser) and the networking time from client to server is saved, whereas the server-side validation is done on the web server and server renders the data into html page and sends back to the client (browser). Thus it is bit slower.
Client side validation can be bypassed by disabling the browser's JavaScript, so we should also validate on the server side because it protects against the malicious user, who can easily bypass our JavaScript and submit dangerous input to the server.
Implementation: Let's create a sample web page to demonstrate the server side validation of asp.net controls.

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head runat="server">
<title></title>
<style type="text/css">
.error
{
border: 1px solid red;
}
.errorstyle
{
font-size: 16px;
line-height: 22px;
}

.errorstyle ul li
{
margin-left: 5px;
color: ‪#‎ff0000‬;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div style="width:300px;">
<fieldset>
<legend>Validate TextBox</legend>
<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
</fieldset>
<fieldset>
<legend>Validate DropDownList</legend>
<asp:DropDownList ID="DropDownList1" runat="server" Width="150px">
<asp:ListItem Value="0" Text="--Select--"></asp:ListItem>
<asp:ListItem Value="1" Text="Asp.Net"></asp:ListItem>
<asp:ListItem Value="2" Text="MVC"></asp:ListItem>
<asp:ListItem Value="3" Text="C#"></asp:ListItem>
<asp:ListItem Value="4" Text="jQuery"></asp:ListItem>
</asp:DropDownList>
</fieldset>
<fieldset>
<legend>Validate CheckBox</legend>
<asp:CheckBox ID="CheckBox1" runat="server" Text="Married" />
</fieldset>
<fieldset>
<legend>Validate CheckBoxList</legend>
<asp:CheckBoxList ID="CheckBoxList1" runat="server" RepeatColumns="2">
<asp:ListItem Value="1" Text="Asp.Net"></asp:ListItem>
<asp:ListItem Value="2" Text="MVC"></asp:ListItem>
<asp:ListItem Value="3" Text="C#"></asp:ListItem>
<asp:ListItem Value="4" Text="jQuery"></asp:ListItem>
</asp:CheckBoxList>
</fieldset>
<fieldset>
<legend>Validate RadioButton</legend>
<asp:RadioButton ID="RadioButton1" runat="server" Text="Married" />
</fieldset>
<fieldset>
<legend>Validate RadioButtonList</legend>
<asp:RadioButtonList ID="RadioButtonList1" runat="server" RepeatColumns="2">
<asp:ListItem Value="1" Text="Asp.Net"></asp:ListItem>
<asp:ListItem Value="2" Text="MVC"></asp:ListItem>
<asp:ListItem Value="3" Text="C#"></asp:ListItem>
<asp:ListItem Value="4" Text="jQuery"></asp:ListItem>
</asp:RadioButtonList>
</fieldset>
<fieldset>
<legend>Validate ListBox</legend>
<asp:ListBox ID="ListBox1" runat="server" Width="150px">
<asp:ListItem Value="1" Text="Asp.Net"></asp:ListItem>
<asp:ListItem Value="2" Text="MVC"></asp:ListItem>
<asp:ListItem Value="3" Text="C#"></asp:ListItem>
<asp:ListItem Value="4" Text="jQuery"></asp:ListItem>
</asp:ListBox>
</fieldset>
<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" />
<div class="errorstyle" >
<asp:Literal ID="ltrlErrorMsg" runat="server"></asp:Literal>
</div>
</div>
</form>
</body>

Asp.Net C# Code to validate controls server side

using System.Text;

protected void btnSubmit_Click(object sender, EventArgs e)
{
if (IsControlsValid())
{
// Form is validated. Write your submit button code here.
}
}

private bool IsControlsValid()
{
bool isValid = true;
StringBuilder sb = new StringBuilder();
sb.Append("<ul>");

//Validate TextBox(can't be left empty)
if (String.IsNullOrEmpty(txtName.Text.Trim()))
{
isValid = false;
txtName.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please enter in TextBox"));
}
else
{
txtName.CssClass = "";
}

//Validate DropDownList(Item must be selected)
if (DropDownList1.SelectedValue == "0")
{
isValid = false;
DropDownList1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select DropDowlist item"));
}
else
{
DropDownList1.CssClass = "";
}

//Validate CheckBox(Must be checked)
if (!CheckBox1.Checked)
{
isValid = false;
CheckBox1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select Checkbox"));
}
else
{
CheckBox1.CssClass = "";
}

//Validate RadioButton(Must be selected)
if (!RadioButton1.Checked)
{
isValid = false;
RadioButton1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select RadioButton"));
}
else
{
RadioButton1.CssClass = "";
}

//Validate CheckBoxList(At least one item must be selected)
foreach (ListItem item in CheckBoxList1.Items)
{
if (item.Selected)
{
isValid = true;
break;
}
else
{
isValid = false;
}
}
if (isValid == false)
{
CheckBoxList1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select at least one option from CheckBoxList"));
}
else
{
CheckBoxList1.CssClass = "";
}

//Validate RadioButton(Item must be selected)
foreach (ListItem item in RadioButtonList1.Items)
{
if (item.Selected)
{
isValid = true;
break;
}
else
{
isValid = false;
}
}
if (isValid == false)
{
RadioButtonList1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select at least one option from RadioButtonList"));
}
else
{
RadioButtonList1.CssClass = "";
}

//Validate ListBox (Item must be selected)
foreach (ListItem item in ListBox1.Items)
{
if (item.Selected)
{
isValid = true;
break;
}
else
{
isValid = false;
}
}
if (isValid == false)
{
ListBox1.CssClass = "error";
sb.Append(String.Format("<li>{0}</li>", "Please select at least one item from ListBox"));
}
else
{
ListBox1.CssClass = "";
}

sb.Append("</ul>");
ltrlErrorMsg.Text = sb.ToString();
return isValid;
}