Idealized situations apart, the gravitational influences acting on an object depend on the object’s position. Take two small objects in the neighbourhood of a massive body: If one of them is closer to the massive body, it will be subject to a stronger gravitational pull. All effects that can be traced back to this variation of gravitational influences from location to location are called tidal effects.
Whenever gravitation is regarded as a force (notably in Newton’s theory of gravity), tidal effects are caused by minute force differences – differences in the strength and direction of the gravitational force at one point in space, as compared to a neighbouring point. These force differences, in turn, are called tidal forces.
The best-known example for tidal effects is the one responsible for their name: High tide and low tide at the sea-shore are caused by position-dependent variations of the gravitational force – very roughly speaking, the oceans on the side of the earth facing the moon are pulled towards that heavenly body more strongly than the solid globe of the earth, and that globe in turn feels a stronger pull than the oceans on the side facing away from the moon.
In the context of general relativity, tidal forces are especially interesting where singularities are concerned – in fact, the theory predicts that regions near a singularity are dominated by very strong and rapidly changing tidal forces (for more information on this, see the spotlight text Of singularities and breadmaking.