RowFilter gets or sets the expression used to filter.
Example:
Filter by code or description
string rowFilter = string.Format("[{0}] like '%{1}%'", "Code", txtSearch.Text);
rowFilter += string.Format(" OR [{0}] like '%{1}%'", "Description", txtSearch.Text);
(DataGridView1.DataSource as DataTable).DefaultView.RowFilter = rowFilter;