Parallel lines are a fixed distance apart and will never meet, no matter how long they are extended. Lines that are parallel have the same gradient. The graphs above, \(y = 2x + 1\) and \(y = 2x - 2\) ...
If lines are perpendicular to each other then \({m_1} \times {m_2} = - 1\) conversely if \({m_1} \times {m_2} = - 1\) then the lines are perpendicular to each other.