One of the mehods to do that:

In Page1,aspx html

<form id="form1" runat="server">
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
        DataSourceID="SqlDataSource1"
        onselectedindexchanged="GridView1_SelectedIndexChanged">
        <Columns>
            <asp:CommandField ShowSelectButton="True" />
            <asp:BoundField DataField="emp_ID" HeaderText="ID" InsertVisible="False"
                ReadOnly="True" SortExpression="emp_ID" />
            <asp:BoundField DataField="emp_Name" HeaderText="Name"
                SortExpression="emp_Name" />
        </Columns>
    </asp:GridView>
    <br />
    <br />
    <asp:SqlDataSource ID="SqlDataSource1" runat="server"
        ConnectionString="<%$ ConnectionStrings:TestConnectionString %>"
        SelectCommand="SELECT [emp_ID], [emp_Name] FROM [emps]"></asp:SqlDataSource>
    <br />
    </form>

in page1.aspx  code behind

 protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
        {
            String myID = GridView1.SelectedRow.Cells[1].Text;
            Page.Session["myID"]= myID;
            Response.Redirect("page2.aspx");

        }

Code behind for page2.aspx

protected void Page_Load(object sender, EventArgs e)
    {
        if (Page.Session["myID"] != null)
        {
            TextBox1.Text = Page.Session["myID"].ToString();
        }
    }