For a DataGridView, how do I get the values from each row?

You can use a foreach to iterate over the DataGridViewRows within the DataGridView and use the column names to retrieve the values from the Cells:

foreach (DataGridViewRow dr in dataGridView.Rows)
{
    string col1 = dr.Cells["Col1"].Value.ToString();
    string col2 = dr.Cells["Col2"].Value.ToString();
    string col3 = dr.Cells["Col3"].Value.ToString();
}