How are lows and highs defined?

The low is a vortex with a vertical axis, the isobars have a cyclonic curvature, the air movement is counterclockwise, the pressure is lowest in the centre of the vortex. The magnitude of the pressure is not the most important; for practical reasons the cyclonic curvature of the isobars causing the wind to converge and forcing the air to move upwards is the most important. As a result, clouds form, then precipitation.
A similar situation occurs in the case of a high, which is also a vortex with a vertical axis; its other features contradict those of a low: anticyclonic curvature of isobars, clockwise air movement. Instead of wind convergence in the area covered by the low, there is wind divergence and descending movements. These lead to the formation of a subsidence inversion (a layer in which the temperature increases with height; when there are no descending movements the air temperature decreases with height). Inversion, more precisely temperature inversion, has an important influence on the vertical development of cumulus clouds.